diff -r f5db9e08effc -r b6815473ee2e Myhill.thy
--- a/Myhill.thy	Mon Jan 24 11:29:55 2011 +0000
+++ b/Myhill.thy	Tue Jan 25 12:14:31 2011 +0000
@@ -1,1570 +1,1492 @@
-theory Myhill
-  imports Main List_Prefix
-begin
-
-section {* Preliminary definitions *}
-
-text {* Sequential composition of two languages @{text "L1"} and @{text "L2"} *}
-definition Seq :: "string set \<Rightarrow> string set \<Rightarrow> string set" ("_ ;; _" [100,100] 100)
-where 
-  "L1 ;; L2 = {s1 @ s2 | s1 s2. s1 \<in> L1 \<and> s2 \<in> L2}"
-
-text {* Transitive closure of language @{text "L"}. *}
-inductive_set
-  Star :: "string set \<Rightarrow> string set" ("_\<star>" [101] 102)
-  for L :: "string set"
-where
-  start[intro]: "[] \<in> L\<star>"
-| step[intro]:  "\<lbrakk>s1 \<in> L; s2 \<in> L\<star>\<rbrakk> \<Longrightarrow> s1@s2 \<in> L\<star>" 
-
-text {* Some properties of operator @{text ";;"}.*}
-
-lemma seq_union_distrib:
-  "(A \<union> B) ;; C = (A ;; C) \<union> (B ;; C)"
-by (auto simp:Seq_def)
-
-lemma seq_intro:
-  "\<lbrakk>x \<in> A; y \<in> B\<rbrakk> \<Longrightarrow> x @ y \<in> A ;; B "
-by (auto simp:Seq_def)
-
-lemma seq_assoc:
-  "(A ;; B) ;; C = A ;; (B ;; C)"
-apply(auto simp:Seq_def)
-apply blast
-by (metis append_assoc)
-
-lemma star_intro1[rule_format]: "x \<in> lang\<star> \<Longrightarrow> \<forall> y. y \<in> lang\<star> \<longrightarrow> x @ y \<in> lang\<star>"
-by (erule Star.induct, auto)
-
-lemma star_intro2: "y \<in> lang \<Longrightarrow> y \<in> lang\<star>"
-by (drule step[of y lang "[]"], auto simp:start)
-
-lemma star_intro3[rule_format]: 
-  "x \<in> lang\<star> \<Longrightarrow> \<forall>y . y \<in> lang \<longrightarrow> x @ y \<in> lang\<star>"
-by (erule Star.induct, auto intro:star_intro2)
-
-lemma star_decom: 
-  "\<lbrakk>x \<in> lang\<star>; x \<noteq> []\<rbrakk> \<Longrightarrow>(\<exists> a b. x = a @ b \<and> a \<noteq> [] \<and> a \<in> lang \<and> b \<in> lang\<star>)"
-by (induct x rule: Star.induct, simp, blast)
-
-lemma star_decom': 
-  "\<lbrakk>x \<in> lang\<star>; x \<noteq> []\<rbrakk> \<Longrightarrow> \<exists>a b. x = a @ b \<and> a \<in> lang\<star> \<and> b \<in> lang"
-apply (induct x rule:Star.induct, simp)
-apply (case_tac "s2 = []")
-apply (rule_tac x = "[]" in exI, rule_tac x = s1 in exI, simp add:start)
-apply (simp, (erule exE| erule conjE)+)
-by (rule_tac x = "s1 @ a" in exI, rule_tac x = b in exI, simp add:step)
-
-text {* Ardens lemma expressed at the level of language, rather than the level of regular expression. *}
-
-theorem ardens_revised:
-  assumes nemp: "[] \<notin> A"
-  shows "(X = X ;; A \<union> B) \<longleftrightarrow> (X = B ;; A\<star>)"
-proof
-  assume eq: "X = B ;; A\<star>"
-  have "A\<star> =  {[]} \<union> A\<star> ;; A" 
-    by (auto simp:Seq_def star_intro3 star_decom')  
-  then have "B ;; A\<star> = B ;; ({[]} \<union> A\<star> ;; A)" 
-    unfolding Seq_def by simp
-  also have "\<dots> = B \<union> B ;; (A\<star> ;; A)"  
-    unfolding Seq_def by auto
-  also have "\<dots> = B \<union> (B ;; A\<star>) ;; A" 
-    by (simp only:seq_assoc)
-  finally show "X = X ;; A \<union> B" 
-    using eq by blast 
-next
-  assume eq': "X = X ;; A \<union> B"
-  hence c1': "\<And> x. x \<in> B \<Longrightarrow> x \<in> X" 
-    and c2': "\<And> x y. \<lbrakk>x \<in> X; y \<in> A\<rbrakk> \<Longrightarrow> x @ y \<in> X" 
-    using Seq_def by auto
-  show "X = B ;; A\<star>" 
-  proof
-    show "B ;; A\<star> \<subseteq> X"
-    proof-
-      { fix x y
-        have "\<lbrakk>y \<in> A\<star>; x \<in> X\<rbrakk> \<Longrightarrow> x @ y \<in> X "
-          apply (induct arbitrary:x rule:Star.induct, simp)
-          by (auto simp only:append_assoc[THEN sym] dest:c2')
-      } thus ?thesis using c1' by (auto simp:Seq_def) 
-    qed
-  next
-    show "X \<subseteq> B ;; A\<star>"
-    proof-
-      { fix x 
-        have "x \<in> X \<Longrightarrow> x \<in> B ;; A\<star>"
-        proof (induct x taking:length rule:measure_induct)
-          fix z
-          assume hyps: 
-            "\<forall>y. length y < length z \<longrightarrow> y \<in> X \<longrightarrow> y \<in> B ;; A\<star>" 
-            and z_in: "z \<in> X"
-          show "z \<in> B ;; A\<star>"
-          proof (cases "z \<in> B")
-            case True thus ?thesis by (auto simp:Seq_def start)
-          next
-            case False hence "z \<in> X ;; A" using eq' z_in by auto
-            then obtain za zb where za_in: "za \<in> X" 
-              and zab: "z = za @ zb \<and> zb \<in> A" and zbne: "zb \<noteq> []" 
-              using nemp unfolding Seq_def by blast
-            from zbne zab have "length za < length z" by auto
-            with za_in hyps have "za \<in> B ;; A\<star>" by blast
-            hence "za @ zb \<in> B ;; A\<star>" using zab 
-              by (clarsimp simp:Seq_def, blast dest:star_intro3)
-            thus ?thesis using zab by simp       
-          qed
-        qed 
-      } thus ?thesis by blast
-    qed
-  qed
-qed
-
-
-text {* The syntax of regular expressions is defined by the datatype @{text "rexp"}. *}
-datatype rexp =
-  NULL
-| EMPTY
-| CHAR char
-| SEQ rexp rexp
-| ALT rexp rexp
-| STAR rexp
-
-
-text {* 
-  The following @{text "L"} is an overloaded operator, where @{text "L(x)"} evaluates to 
-  the language represented by the syntactic object @{text "x"}.
-*}
-consts L:: "'a \<Rightarrow> string set"
-
-
-text {* 
-  The @{text "L(rexp)"} for regular expression @{text "rexp"} is defined by the 
-  following overloading function @{text "L_rexp"}.
-*}
-overloading L_rexp \<equiv> "L::  rexp \<Rightarrow> string set"
-begin
-fun
-  L_rexp :: "rexp \<Rightarrow> string set"
-where
-    "L_rexp (NULL) = {}"
-  | "L_rexp (EMPTY) = {[]}"
-  | "L_rexp (CHAR c) = {[c]}"
-  | "L_rexp (SEQ r1 r2) = (L_rexp r1) ;; (L_rexp r2)"
-  | "L_rexp (ALT r1 r2) = (L_rexp r1) \<union> (L_rexp r2)"
-  | "L_rexp (STAR r) = (L_rexp r)\<star>"
-end
-
-text {*
-  To obtain equational system out of finite set of equivalent classes, a fold operation
-  on finite set @{text "folds"} is defined. The use of @{text "SOME"} makes @{text "fold"}
-  more robust than the @{text "fold"} in Isabelle library. The expression @{text "folds f"}
-  makes sense when @{text "f"} is not @{text "associative"} and @{text "commutitive"},
-  while @{text "fold f"} does not.  
-*}
-
-definition 
-  folds :: "('a \<Rightarrow> 'b \<Rightarrow> 'b) \<Rightarrow> 'b \<Rightarrow> 'a set \<Rightarrow> 'b"
-where
-  "folds f z S \<equiv> SOME x. fold_graph f z S x"
-
-text {* 
-  The following lemma assures that the arbitrary choice made by the @{text "SOME"} in @{text "folds"}
-  does not affect the @{text "L"}-value of the resultant regular expression. 
-  *}
-lemma folds_alt_simp [simp]:
-  "finite rs \<Longrightarrow> L (folds ALT NULL rs) = \<Union> (L ` rs)"
-apply (rule set_ext, simp add:folds_def)
-apply (rule someI2_ex, erule finite_imp_fold_graph)
-by (erule fold_graph.induct, auto)
-
-(* Just a technical lemma. *)
-lemma [simp]:
-  shows "(x, y) \<in> {(x, y). P x y} \<longleftrightarrow> P x y"
-by simp
-
-
-text {*
-  @{text "\<approx>L"} is an equivalent class defined by language @{text "Lang"}.
-*}
-definition
-  str_eq_rel ("\<approx>_")
-where
-  "\<approx>Lang \<equiv> {(x, y).  (\<forall>z. x @ z \<in> Lang \<longleftrightarrow> y @ z \<in> Lang)}"
-
-text {* 
-  Among equivlant clases of @{text "\<approx>Lang"}, the set @{text "finals(Lang)"} singles out 
-  those which contains strings from @{text "Lang"}.
-*}
-
-definition 
-   "finals Lang \<equiv> {\<approx>Lang `` {x} | x . x \<in> Lang}"
-
-text {* 
-  The following lemma show the relationshipt between @{text "finals(Lang)"} and @{text "Lang"}.
-*}
-lemma lang_is_union_of_finals: 
-  "Lang = \<Union> finals(Lang)"
-proof 
-  show "Lang \<subseteq> \<Union> (finals Lang)"
-  proof
-    fix x
-    assume "x \<in> Lang"   
-    thus "x \<in> \<Union> (finals Lang)"
-      apply (simp add:finals_def, rule_tac x = "(\<approx>Lang) `` {x}" in exI)
-      by (auto simp:Image_def str_eq_rel_def)    
-  qed
-next
-  show "\<Union> (finals Lang) \<subseteq> Lang"
-    apply (clarsimp simp:finals_def str_eq_rel_def)
-    by (drule_tac x = "[]" in spec, auto)
-qed
-
-section {* Direction @{text "finite partition \<Rightarrow> regular language"}*}
-
-text {* 
-  The relationship between equivalent classes can be described by an
-  equational system.
-  For example, in equational system \eqref{example_eqns},  $X_0, X_1$ are equivalent 
-  classes. The first equation says every string in $X_0$ is obtained either by
-  appending one $b$ to a string in $X_0$ or by appending one $a$ to a string in
-  $X_1$ or just be an empty string (represented by the regular expression $\lambda$). Similary,
-  the second equation tells how the strings inside $X_1$ are composed.
-  \begin{equation}\label{example_eqns}
-    \begin{aligned}
-      X_0 & = X_0 b + X_1 a + \lambda \\
-      X_1 & = X_0 a + X_1 b
-    \end{aligned}
-  \end{equation}
-  The summands on the right hand side is represented by the following data type
-  @{text "rhs_item"}, mnemonic for 'right hand side item'.
-  Generally, there are two kinds of right hand side items, one kind corresponds to
-  pure regular expressions, like the $\lambda$ in \eqref{example_eqns}, the other kind corresponds to
-  transitions from one one equivalent class to another, like the $X_0 b, X_1 a$ etc.
-  *}
-
-datatype rhs_item = 
-   Lam "rexp"                           (* Lambda *)
- | Trn "(string set)" "rexp"              (* Transition *)
-
-text {*
-  In this formalization, pure regular expressions like $\lambda$ is 
-  repsented by @{text "Lam(EMPTY)"}, while transitions like $X_0 a$ is represented by $Trn~X_0~(CHAR~a)$.
-  *}
-
-text {*
-  The functions @{text "the_r"} and @{text "the_Trn"} are used to extract
-  subcomponents from right hand side items.
-  *}
-
-fun the_r :: "rhs_item \<Rightarrow> rexp"
-where "the_r (Lam r) = r"
-
-fun the_Trn:: "rhs_item \<Rightarrow> (string set \<times> rexp)"
-where "the_Trn (Trn Y r) = (Y, r)"
-
-text {*
-  Every right hand side item @{text "itm"} defines a string set given 
-  @{text "L(itm)"}, defined as:
-*}
-overloading L_rhs_e \<equiv> "L:: rhs_item \<Rightarrow> string set"
-begin
-  fun L_rhs_e:: "rhs_item \<Rightarrow> string set"
-  where
-     "L_rhs_e (Lam r) = L r" |
-     "L_rhs_e (Trn X r) = X ;; L r"
-end
-
-text {*
-  The right hand side of every equation is represented by a set of
-  items. The string set defined by such a set @{text "itms"} is given
-  by @{text "L(itms)"}, defined as:
-*}
-
-overloading L_rhs \<equiv> "L:: rhs_item set \<Rightarrow> string set"
-begin
-   fun L_rhs:: "rhs_item set \<Rightarrow> string set"
-   where "L_rhs rhs = \<Union> (L ` rhs)"
-end
-
-text {* 
-  Given a set of equivalent classses @{text "CS"} and one equivalent class @{text "X"} among
-  @{text "CS"}, the term @{text "init_rhs CS X"} is used to extract the right hand side of
-  the equation describing the formation of @{text "X"}. The definition of @{text "init_rhs"}
-  is:
-  *}
-
-definition
-  "init_rhs CS X \<equiv>  
-      if ([] \<in> X) then 
-          {Lam(EMPTY)} \<union> {Trn Y (CHAR c) | Y c. Y \<in> CS \<and> Y ;; {[c]} \<subseteq> X}
-      else 
-          {Trn Y (CHAR c)| Y c. Y \<in> CS \<and> Y ;; {[c]} \<subseteq> X}"
-
-text {*
-  In the definition of @{text "init_rhs"}, the term 
-  @{text "{Trn Y (CHAR c)| Y c. Y \<in> CS \<and> Y ;; {[c]} \<subseteq> X}"} appearing on both branches
-  describes the formation of strings in @{text "X"} out of transitions, while 
-  the term @{text "{Lam(EMPTY)}"} describes the empty string which is intrinsically contained in
-  @{text "X"} rather than by transition. This @{text "{Lam(EMPTY)}"} corresponds to 
-  the $\lambda$ in \eqref{example_eqns}.
-
-  With the help of @{text "init_rhs"}, the equitional system descrbing the formation of every
-  equivalent class inside @{text "CS"} is given by the following @{text "eqs(CS)"}.
-  *}
-
-definition "eqs CS \<equiv> {(X, init_rhs CS X) | X.  X \<in> CS}"
-(************ arden's lemma variation ********************)
-
-text {* 
-  The following @{text "items_of rhs X"} returns all @{text "X"}-items in @{text "rhs"}.
-  *}
-definition
-  "items_of rhs X \<equiv> {Trn X r | r. (Trn X r) \<in> rhs}"
-
-text {* 
-  The following @{text "rexp_of rhs X"} combines all regular expressions in @{text "X"}-items
-  using @{text "ALT"} to form a single regular expression. 
-  It will be used later to implement @{text "arden_variate"} and @{text "rhs_subst"}.
-  *}
-
-definition 
-  "rexp_of rhs X \<equiv> folds ALT NULL ((snd o the_Trn) ` items_of rhs X)"
-
-text {* 
-  The following @{text "lam_of rhs"} returns all pure regular expression items in @{text "rhs"}.
-  *}
-
-definition
-  "lam_of rhs \<equiv> {Lam r | r. Lam r \<in> rhs}"
-
-text {*
-  The following @{text "rexp_of_lam rhs"} combines pure regular expression items in @{text "rhs"}
-  using @{text "ALT"} to form a single regular expression. 
-  When all variables inside @{text "rhs"} are eliminated, @{text "rexp_of_lam rhs"}
-  is used to compute compute the regular expression corresponds to @{text "rhs"}.
-  *}
-
-definition
-  "rexp_of_lam rhs \<equiv> folds ALT NULL (the_r ` lam_of rhs)"
-
-text {*
-  The following @{text "attach_rexp rexp' itm"} attach 
-  the regular expression @{text "rexp'"} to
-  the right of right hand side item @{text "itm"}.
-  *}
-
-fun attach_rexp :: "rexp \<Rightarrow> rhs_item \<Rightarrow> rhs_item"
-where
-  "attach_rexp rexp' (Lam rexp)   = Lam (SEQ rexp rexp')"
-| "attach_rexp rexp' (Trn X rexp) = Trn X (SEQ rexp rexp')"
-
-text {* 
-  The following @{text "append_rhs_rexp rhs rexp"} attaches 
-  @{text "rexp"} to every item in @{text "rhs"}.
-  *}
-
-definition
-  "append_rhs_rexp rhs rexp \<equiv> (attach_rexp rexp) ` rhs"
-
-text {*
-  With the help of the two functions immediately above, Ardens'
-  transformation on right hand side @{text "rhs"} is implemented
-  by the following function @{text "arden_variate X rhs"}.
-  After this transformation, the recursive occurent of @{text "X"}
-  in @{text "rhs"} will be eliminated, while the 
-  string set defined by @{text "rhs"} is kept unchanged.
-  *}
-definition 
-  "arden_variate X rhs \<equiv> 
-        append_rhs_rexp (rhs - items_of rhs X) (STAR (rexp_of rhs X))"
-
-
-(*********** substitution of ES *************)
-
-text {* 
-  Suppose the equation defining @{text "X"} is $X = xrhs$,
-  the purpose of @{text "rhs_subst"} is to substitute all occurences of @{text "X"} in
-  @{text "rhs"} by @{text "xrhs"}.
-  A litte thought may reveal that the final result
-  should be: first append $(a_1 | a_2 | \ldots | a_n)$ to every item of @{text "xrhs"} and then
-  union the result with all non-@{text "X"}-items of @{text "rhs"}.
- *}
-definition 
-  "rhs_subst rhs X xrhs \<equiv> 
-        (rhs - (items_of rhs X)) \<union> (append_rhs_rexp xrhs (rexp_of rhs X))"
-
-text {*
-  Suppose the equation defining @{text "X"} is $X = xrhs$, the follwing
-  @{text "eqs_subst ES X xrhs"} substitute @{text "xrhs"} into every equation
-  of the equational system @{text "ES"}.
-  *}
-
-definition
-  "eqs_subst ES X xrhs \<equiv> {(Y, rhs_subst yrhs X xrhs) | Y yrhs. (Y, yrhs) \<in> ES}"
-
-text {*
-  The computation of regular expressions for equivalent classes is accomplished
-  using a iteration principle given by the following lemma.
-  *}
-
-lemma wf_iter [rule_format]: 
-  fixes f
-  assumes step: "\<And> e. \<lbrakk>P e; \<not> Q e\<rbrakk> \<Longrightarrow> (\<exists> e'. P e' \<and>  (f(e'), f(e)) \<in> less_than)"
-  shows pe:     "P e \<longrightarrow> (\<exists> e'. P e' \<and>  Q e')"
-proof(induct e rule: wf_induct 
-           [OF wf_inv_image[OF wf_less_than, where f = "f"]], clarify)
-  fix x 
-  assume h [rule_format]: 
-    "\<forall>y. (y, x) \<in> inv_image less_than f \<longrightarrow> P y \<longrightarrow> (\<exists>e'. P e' \<and> Q e')"
-    and px: "P x"
-  show "\<exists>e'. P e' \<and> Q e'"
-  proof(cases "Q x")
-    assume "Q x" with px show ?thesis by blast
-  next
-    assume nq: "\<not> Q x"
-    from step [OF px nq]
-    obtain e' where pe': "P e'" and ltf: "(f e', f x) \<in> less_than" by auto
-    show ?thesis
-    proof(rule h)
-      from ltf show "(e', x) \<in> inv_image less_than f" 
-	by (simp add:inv_image_def)
-    next
-      from pe' show "P e'" .
-    qed
-  qed
-qed
-
-text {*
-  The @{text "P"} in lemma @{text "wf_iter"} is an invaiant kept throughout the iteration procedure.
-  The particular invariant used to solve our problem is defined by function @{text "Inv(ES)"},
-  an invariant over equal system @{text "ES"}.
-  Every definition starting next till @{text "Inv"} stipulates a property to be satisfied by @{text "ES"}.
-*}
-
-text {* 
-  Every variable is defined at most onece in @{text "ES"}.
-  *}
-definition 
-  "distinct_equas ES \<equiv> 
-            \<forall> X rhs rhs'. (X, rhs) \<in> ES \<and> (X, rhs') \<in> ES \<longrightarrow> rhs = rhs'"
-text {* 
-  Every equation in @{text "ES"} (represented by @{text "(X, rhs)"}) is valid, i.e. @{text "(X = L rhs)"}.
-  *}
-definition 
-  "valid_eqns ES \<equiv> \<forall> X rhs. (X, rhs) \<in> ES \<longrightarrow> (X = L rhs)"
-
-text {*
-  @{text "rhs_nonempty rhs"} requires regular expressions occuring in transitional 
-  items of @{text "rhs"} does not contain empty string. This is necessary for
-  the application of Arden's transformation to @{text "rhs"}.
-  *}
-definition 
-  "rhs_nonempty rhs \<equiv> (\<forall> Y r. Trn Y r \<in> rhs \<longrightarrow> [] \<notin> L r)"
-
-text {*
-  @{text "ardenable ES"} requires that Arden's transformation is applicable
-  to every equation of equational system @{text "ES"}.
-  *}
-definition 
-  "ardenable ES \<equiv> \<forall> X rhs. (X, rhs) \<in> ES \<longrightarrow> rhs_nonempty rhs"
-
-(* The following non_empty seems useless. *)
-definition 
-  "non_empty ES \<equiv> \<forall> X rhs. (X, rhs) \<in> ES \<longrightarrow> X \<noteq> {}"
-
-text {*
-  The following @{text "finite_rhs ES"} requires every equation in @{text "rhs"} be finite.
-  *}
-definition
-  "finite_rhs ES \<equiv> \<forall> X rhs. (X, rhs) \<in> ES \<longrightarrow> finite rhs"
-
-text {*
-  The following @{text "classes_of rhs"} returns all variables (or equivalent classes)
-  occuring in @{text "rhs"}.
-  *}
-definition 
-  "classes_of rhs \<equiv> {X. \<exists> r. Trn X r \<in> rhs}"
-
-text {*
-  The following @{text "lefts_of ES"} returns all variables 
-  defined by equational system @{text "ES"}.
-  *}
-definition
-  "lefts_of ES \<equiv> {Y | Y yrhs. (Y, yrhs) \<in> ES}"
-
-text {*
-  The following @{text "self_contained ES"} requires that every
-  variable occuring on the right hand side of equations is already defined by some
-  equation in @{text "ES"}.
-  *}
-definition 
-  "self_contained ES \<equiv> \<forall> (X, xrhs) \<in> ES. classes_of xrhs \<subseteq> lefts_of ES"
-
-
-text {*
-  The invariant @{text "Inv(ES)"} is obtained by conjunctioning all the previous
-  defined constaints on @{text "ES"}.
-  *}
-definition 
-  "Inv ES \<equiv> valid_eqns ES \<and> finite ES \<and> distinct_equas ES \<and> ardenable ES \<and> 
-                non_empty ES \<and> finite_rhs ES \<and> self_contained ES"
-
-subsection {* Proof for this direction *}
-
-
-
-text {*
-  The following are some basic properties of the above definitions.
-*}
-
-lemma L_rhs_union_distrib:
-  " L (A::rhs_item set) \<union> L B = L (A \<union> B)"
-by simp
-
-lemma finite_snd_Trn:
-  assumes finite:"finite rhs"
-  shows "finite {r\<^isub>2. Trn Y r\<^isub>2 \<in> rhs}" (is "finite ?B")
-proof-
-  def rhs' \<equiv> "{e \<in> rhs. \<exists> r. e = Trn Y r}"
-  have "?B = (snd o the_Trn) ` rhs'" using rhs'_def by (auto simp:image_def)
-  moreover have "finite rhs'" using finite rhs'_def by auto
-  ultimately show ?thesis by simp
-qed
-
-lemma rexp_of_empty:
-  assumes finite:"finite rhs"
-  and nonempty:"rhs_nonempty rhs"
-  shows "[] \<notin> L (rexp_of rhs X)"
-using finite nonempty rhs_nonempty_def
-by (drule_tac finite_snd_Trn[where Y = X], auto simp:rexp_of_def items_of_def)
-
-lemma [intro!]:
-  "P (Trn X r) \<Longrightarrow> (\<exists>a. (\<exists>r. a = Trn X r \<and> P a))" by auto
-
-lemma finite_items_of:
-  "finite rhs \<Longrightarrow> finite (items_of rhs X)"
-by (auto simp:items_of_def intro:finite_subset)
-
-lemma lang_of_rexp_of:
-  assumes finite:"finite rhs"
-  shows "L (items_of rhs X) = X ;; (L (rexp_of rhs X))"
-proof -
-  have "finite ((snd \<circ> the_Trn) ` items_of rhs X)" using finite_items_of[OF finite] by auto
-  thus ?thesis
-    apply (auto simp:rexp_of_def Seq_def items_of_def)
-    apply (rule_tac x = s1 in exI, rule_tac x = s2 in exI, auto)
-    by (rule_tac x= "Trn X r" in exI, auto simp:Seq_def)
-qed
-
-lemma rexp_of_lam_eq_lam_set:
-  assumes finite: "finite rhs"
-  shows "L (rexp_of_lam rhs) = L (lam_of rhs)"
-proof -
-  have "finite (the_r ` {Lam r |r. Lam r \<in> rhs})" using finite
-    by (rule_tac finite_imageI, auto intro:finite_subset)
-  thus ?thesis by (auto simp:rexp_of_lam_def lam_of_def)
-qed
-
-lemma [simp]:
-  " L (attach_rexp r xb) = L xb ;; L r"
-apply (cases xb, auto simp:Seq_def)
-by (rule_tac x = "s1 @ s1a" in exI, rule_tac x = s2a in exI,auto simp:Seq_def)
-
-lemma lang_of_append_rhs:
-  "L (append_rhs_rexp rhs r) = L rhs ;; L r"
-apply (auto simp:append_rhs_rexp_def image_def)
-apply (auto simp:Seq_def)
-apply (rule_tac x = "L xb ;; L r" in exI, auto simp add:Seq_def)
-by (rule_tac x = "attach_rexp r xb" in exI, auto simp:Seq_def)
-
-lemma classes_of_union_distrib:
-  "classes_of A \<union> classes_of B = classes_of (A \<union> B)"
-by (auto simp add:classes_of_def)
-
-lemma lefts_of_union_distrib:
-  "lefts_of A \<union> lefts_of B = lefts_of (A \<union> B)"
-by (auto simp:lefts_of_def)
-
-
-text {*
-  The following several lemmas until @{text "init_ES_satisfy_Inv"} are
-  to prove that initial equational system satisfies invariant @{text "Inv"}.
-  *}
-
-lemma defined_by_str:
-  "\<lbrakk>s \<in> X; X \<in> UNIV // (\<approx>Lang)\<rbrakk> \<Longrightarrow> X = (\<approx>Lang) `` {s}"
-by (auto simp:quotient_def Image_def str_eq_rel_def)
-
-lemma every_eqclass_has_transition:
-  assumes has_str: "s @ [c] \<in> X"
-  and     in_CS:   "X \<in> UNIV // (\<approx>Lang)"
-  obtains Y where "Y \<in> UNIV // (\<approx>Lang)" and "Y ;; {[c]} \<subseteq> X" and "s \<in> Y"
-proof -
-  def Y \<equiv> "(\<approx>Lang) `` {s}"
-  have "Y \<in> UNIV // (\<approx>Lang)" 
-    unfolding Y_def quotient_def by auto
-  moreover
-  have "X = (\<approx>Lang) `` {s @ [c]}" 
-    using has_str in_CS defined_by_str by blast
-  then have "Y ;; {[c]} \<subseteq> X" 
-    unfolding Y_def Image_def Seq_def
-    unfolding str_eq_rel_def
-    by clarsimp
-  moreover
-  have "s \<in> Y" unfolding Y_def 
-    unfolding Image_def str_eq_rel_def by simp
-  ultimately show thesis by (blast intro: that)
-qed
-
-lemma l_eq_r_in_eqs:
-  assumes X_in_eqs: "(X, xrhs) \<in> (eqs (UNIV // (\<approx>Lang)))"
-  shows "X = L xrhs"
-proof 
-  show "X \<subseteq> L xrhs"
-  proof
-    fix x
-    assume "(1)": "x \<in> X"
-    show "x \<in> L xrhs"          
-    proof (cases "x = []")
-      assume empty: "x = []"
-      thus ?thesis using X_in_eqs "(1)"
-        by (auto simp:eqs_def init_rhs_def)
-    next
-      assume not_empty: "x \<noteq> []"
-      then obtain clist c where decom: "x = clist @ [c]"
-        by (case_tac x rule:rev_cases, auto)
-      have "X \<in> UNIV // (\<approx>Lang)" using X_in_eqs by (auto simp:eqs_def)
-      then obtain Y 
-        where "Y \<in> UNIV // (\<approx>Lang)" 
-        and "Y ;; {[c]} \<subseteq> X"
-        and "clist \<in> Y"
-        using decom "(1)" every_eqclass_has_transition by blast
-      hence 
-        "x \<in> L {Trn Y (CHAR c)| Y c. Y \<in> UNIV // (\<approx>Lang) \<and> Y ;; {[c]} \<subseteq> X}"
-        using "(1)" decom
-        by (simp, rule_tac x = "Trn Y (CHAR c)" in exI, simp add:Seq_def)
-      thus ?thesis using X_in_eqs "(1)"
-        by (simp add:eqs_def init_rhs_def)
-    qed
-  qed
-next
-  show "L xrhs \<subseteq> X" using X_in_eqs
-    by (auto simp:eqs_def init_rhs_def) 
-qed
-
-lemma finite_init_rhs: 
-  assumes finite: "finite CS"
-  shows "finite (init_rhs CS X)"
-proof-
-  have "finite {Trn Y (CHAR c) |Y c. Y \<in> CS \<and> Y ;; {[c]} \<subseteq> X}" (is "finite ?A")
-  proof -
-    def S \<equiv> "{(Y, c)| Y c. Y \<in> CS \<and> Y ;; {[c]} \<subseteq> X}" 
-    def h \<equiv> "\<lambda> (Y, c). Trn Y (CHAR c)"
-    have "finite (CS \<times> (UNIV::char set))" using finite by auto
-    hence "finite S" using S_def 
-      by (rule_tac B = "CS \<times> UNIV" in finite_subset, auto)
-    moreover have "?A = h ` S" by (auto simp: S_def h_def image_def)
-    ultimately show ?thesis 
-      by auto
-  qed
-  thus ?thesis by (simp add:init_rhs_def)
-qed
-
-lemma init_ES_satisfy_Inv:
-  assumes finite_CS: "finite (UNIV // (\<approx>Lang))"
-  shows "Inv (eqs (UNIV // (\<approx>Lang)))"
-proof -
-  have "finite (eqs (UNIV // (\<approx>Lang)))" using finite_CS
-    by (simp add:eqs_def)
-  moreover have "distinct_equas (eqs (UNIV // (\<approx>Lang)))"     
-    by (simp add:distinct_equas_def eqs_def)
-  moreover have "ardenable (eqs (UNIV // (\<approx>Lang)))"
-    by (auto simp add:ardenable_def eqs_def init_rhs_def rhs_nonempty_def del:L_rhs.simps)
-  moreover have "valid_eqns (eqs (UNIV // (\<approx>Lang)))"
-    using l_eq_r_in_eqs by (simp add:valid_eqns_def)
-  moreover have "non_empty (eqs (UNIV // (\<approx>Lang)))"
-    by (auto simp:non_empty_def eqs_def quotient_def Image_def str_eq_rel_def)
-  moreover have "finite_rhs (eqs (UNIV // (\<approx>Lang)))"
-    using finite_init_rhs[OF finite_CS] 
-    by (auto simp:finite_rhs_def eqs_def)
-  moreover have "self_contained (eqs (UNIV // (\<approx>Lang)))"
-    by (auto simp:self_contained_def eqs_def init_rhs_def classes_of_def lefts_of_def)
-  ultimately show ?thesis by (simp add:Inv_def)
-qed
-
-text {*
-  From this point until @{text "iteration_step"}, we are trying to prove 
-  that there exists iteration steps which keep @{text "Inv(ES)"} while
-  decreasing the size of @{text "ES"} with every iteration.
-  *}
-lemma arden_variate_keeps_eq:
-  assumes l_eq_r: "X = L rhs"
-  and not_empty: "[] \<notin> L (rexp_of rhs X)"
-  and finite: "finite rhs"
-  shows "X = L (arden_variate X rhs)"
-proof -
-  def A \<equiv> "L (rexp_of rhs X)"
-  def b \<equiv> "rhs - items_of rhs X"
-  def B \<equiv> "L b" 
-  have "X = B ;; A\<star>"
-  proof-
-    have "rhs = items_of rhs X \<union> b" by (auto simp:b_def items_of_def)
-    hence "L rhs = L(items_of rhs X \<union> b)" by simp
-    hence "L rhs = L(items_of rhs X) \<union> B" by (simp only:L_rhs_union_distrib B_def)
-    with lang_of_rexp_of
-    have "L rhs = X ;; A \<union> B " using finite by (simp only:B_def b_def A_def)
-    thus ?thesis
-      using l_eq_r not_empty
-      apply (drule_tac B = B and X = X in ardens_revised)
-      by (auto simp:A_def simp del:L_rhs.simps)
-  qed
-  moreover have "L (arden_variate X rhs) = (B ;; A\<star>)" (is "?L = ?R")
-    by (simp only:arden_variate_def L_rhs_union_distrib lang_of_append_rhs 
-                  B_def A_def b_def L_rexp.simps seq_union_distrib)
-   ultimately show ?thesis by simp
-qed 
-
-lemma append_keeps_finite:
-  "finite rhs \<Longrightarrow> finite (append_rhs_rexp rhs r)"
-by (auto simp:append_rhs_rexp_def)
-
-lemma arden_variate_keeps_finite:
-  "finite rhs \<Longrightarrow> finite (arden_variate X rhs)"
-by (auto simp:arden_variate_def append_keeps_finite)
-
-lemma append_keeps_nonempty:
-  "rhs_nonempty rhs \<Longrightarrow> rhs_nonempty (append_rhs_rexp rhs r)"
-apply (auto simp:rhs_nonempty_def append_rhs_rexp_def)
-by (case_tac x, auto simp:Seq_def)
-
-lemma nonempty_set_sub:
-  "rhs_nonempty rhs \<Longrightarrow> rhs_nonempty (rhs - A)"
-by (auto simp:rhs_nonempty_def)
-
-lemma nonempty_set_union:
-  "\<lbrakk>rhs_nonempty rhs; rhs_nonempty rhs'\<rbrakk> \<Longrightarrow> rhs_nonempty (rhs \<union> rhs')"
-by (auto simp:rhs_nonempty_def)
-
-lemma arden_variate_keeps_nonempty:
-  "rhs_nonempty rhs \<Longrightarrow> rhs_nonempty (arden_variate X rhs)"
-by (simp only:arden_variate_def append_keeps_nonempty nonempty_set_sub)
-
-
-lemma rhs_subst_keeps_nonempty:
-  "\<lbrakk>rhs_nonempty rhs; rhs_nonempty xrhs\<rbrakk> \<Longrightarrow> rhs_nonempty (rhs_subst rhs X xrhs)"
-by (simp only:rhs_subst_def append_keeps_nonempty  nonempty_set_union nonempty_set_sub)
-
-lemma rhs_subst_keeps_eq:
-  assumes substor: "X = L xrhs"
-  and finite: "finite rhs"
-  shows "L (rhs_subst rhs X xrhs) = L rhs" (is "?Left = ?Right")
-proof-
-  def A \<equiv> "L (rhs - items_of rhs X)"
-  have "?Left = A \<union> L (append_rhs_rexp xrhs (rexp_of rhs X))"
-    by (simp only:rhs_subst_def L_rhs_union_distrib A_def)
-  moreover have "?Right = A \<union> L (items_of rhs X)"
-  proof-
-    have "rhs = (rhs - items_of rhs X) \<union> (items_of rhs X)" by (auto simp:items_of_def)
-    thus ?thesis by (simp only:L_rhs_union_distrib A_def)
-  qed
-  moreover have "L (append_rhs_rexp xrhs (rexp_of rhs X)) = L (items_of rhs X)" 
-    using finite substor  by (simp only:lang_of_append_rhs lang_of_rexp_of)
-  ultimately show ?thesis by simp
-qed
-
-lemma rhs_subst_keeps_finite_rhs:
-  "\<lbrakk>finite rhs; finite yrhs\<rbrakk> \<Longrightarrow> finite (rhs_subst rhs Y yrhs)"
-by (auto simp:rhs_subst_def append_keeps_finite)
-
-lemma eqs_subst_keeps_finite:
-  assumes finite:"finite (ES:: (string set \<times> rhs_item set) set)"
-  shows "finite (eqs_subst ES Y yrhs)"
-proof -
-  have "finite {(Ya, rhs_subst yrhsa Y yrhs) |Ya yrhsa. (Ya, yrhsa) \<in> ES}" 
-                                                                  (is "finite ?A")
-  proof-
-    def eqns' \<equiv> "{((Ya::string set), yrhsa)| Ya yrhsa. (Ya, yrhsa) \<in> ES}"
-    def h \<equiv> "\<lambda> ((Ya::string set), yrhsa). (Ya, rhs_subst yrhsa Y yrhs)"
-    have "finite (h ` eqns')" using finite h_def eqns'_def by auto
-    moreover have "?A = h ` eqns'" by (auto simp:h_def eqns'_def)
-    ultimately show ?thesis by auto      
-  qed
-  thus ?thesis by (simp add:eqs_subst_def)
-qed
-
-lemma eqs_subst_keeps_finite_rhs:
-  "\<lbrakk>finite_rhs ES; finite yrhs\<rbrakk> \<Longrightarrow> finite_rhs (eqs_subst ES Y yrhs)"
-by (auto intro:rhs_subst_keeps_finite_rhs simp add:eqs_subst_def finite_rhs_def)
-
-lemma append_rhs_keeps_cls:
-  "classes_of (append_rhs_rexp rhs r) = classes_of rhs"
-apply (auto simp:classes_of_def append_rhs_rexp_def)
-apply (case_tac xa, auto simp:image_def)
-by (rule_tac x = "SEQ ra r" in exI, rule_tac x = "Trn x ra" in bexI, simp+)
-
-lemma arden_variate_removes_cl:
-  "classes_of (arden_variate Y yrhs) = classes_of yrhs - {Y}"
-apply (simp add:arden_variate_def append_rhs_keeps_cls items_of_def)
-by (auto simp:classes_of_def)
-
-lemma lefts_of_keeps_cls:
-  "lefts_of (eqs_subst ES Y yrhs) = lefts_of ES"
-by (auto simp:lefts_of_def eqs_subst_def)
-
-lemma rhs_subst_updates_cls:
-  "X \<notin> classes_of xrhs \<Longrightarrow> 
-      classes_of (rhs_subst rhs X xrhs) = classes_of rhs \<union> classes_of xrhs - {X}"
-apply (simp only:rhs_subst_def append_rhs_keeps_cls 
-                              classes_of_union_distrib[THEN sym])
-by (auto simp:classes_of_def items_of_def)
-
-lemma eqs_subst_keeps_self_contained:
-  fixes Y
-  assumes sc: "self_contained (ES \<union> {(Y, yrhs)})" (is "self_contained ?A")
-  shows "self_contained (eqs_subst ES Y (arden_variate Y yrhs))" 
-                                                   (is "self_contained ?B")
-proof-
-  { fix X xrhs'
-    assume "(X, xrhs') \<in> ?B"
-    then obtain xrhs 
-      where xrhs_xrhs': "xrhs' = rhs_subst xrhs Y (arden_variate Y yrhs)"
-      and X_in: "(X, xrhs) \<in> ES" by (simp add:eqs_subst_def, blast)    
-    have "classes_of xrhs' \<subseteq> lefts_of ?B"
-    proof-
-      have "lefts_of ?B = lefts_of ES" by (auto simp add:lefts_of_def eqs_subst_def)
-      moreover have "classes_of xrhs' \<subseteq> lefts_of ES"
-      proof-
-        have "classes_of xrhs' \<subseteq> 
-                        classes_of xrhs \<union> classes_of (arden_variate Y yrhs) - {Y}"
-        proof-
-          have "Y \<notin> classes_of (arden_variate Y yrhs)" 
-            using arden_variate_removes_cl by simp
-          thus ?thesis using xrhs_xrhs' by (auto simp:rhs_subst_updates_cls)
-        qed
-        moreover have "classes_of xrhs \<subseteq> lefts_of ES \<union> {Y}" using X_in sc
-          apply (simp only:self_contained_def lefts_of_union_distrib[THEN sym])
-          by (drule_tac x = "(X, xrhs)" in bspec, auto simp:lefts_of_def)
-        moreover have "classes_of (arden_variate Y yrhs) \<subseteq> lefts_of ES \<union> {Y}" 
-          using sc 
-          by (auto simp add:arden_variate_removes_cl self_contained_def lefts_of_def)
-        ultimately show ?thesis by auto
-      qed
-      ultimately show ?thesis by simp
-    qed
-  } thus ?thesis by (auto simp only:eqs_subst_def self_contained_def)
-qed
-
-lemma eqs_subst_satisfy_Inv:
-  assumes Inv_ES: "Inv (ES \<union> {(Y, yrhs)})"
-  shows "Inv (eqs_subst ES Y (arden_variate Y yrhs))"
-proof -  
-  have finite_yrhs: "finite yrhs" 
-    using Inv_ES by (auto simp:Inv_def finite_rhs_def)
-  have nonempty_yrhs: "rhs_nonempty yrhs" 
-    using Inv_ES by (auto simp:Inv_def ardenable_def)
-  have Y_eq_yrhs: "Y = L yrhs" 
-    using Inv_ES by (simp only:Inv_def valid_eqns_def, blast)
-  have "distinct_equas (eqs_subst ES Y (arden_variate Y yrhs))" 
-    using Inv_ES
-    by (auto simp:distinct_equas_def eqs_subst_def Inv_def)
-  moreover have "finite (eqs_subst ES Y (arden_variate Y yrhs))" 
-    using Inv_ES by (simp add:Inv_def eqs_subst_keeps_finite)
-  moreover have "finite_rhs (eqs_subst ES Y (arden_variate Y yrhs))"
-  proof-
-    have "finite_rhs ES" using Inv_ES 
-      by (simp add:Inv_def finite_rhs_def)
-    moreover have "finite (arden_variate Y yrhs)"
-    proof -
-      have "finite yrhs" using Inv_ES 
-        by (auto simp:Inv_def finite_rhs_def)
-      thus ?thesis using arden_variate_keeps_finite by simp
-    qed
-    ultimately show ?thesis 
-      by (simp add:eqs_subst_keeps_finite_rhs)
-  qed
-  moreover have "ardenable (eqs_subst ES Y (arden_variate Y yrhs))"
-  proof - 
-    { fix X rhs
-      assume "(X, rhs) \<in> ES"
-      hence "rhs_nonempty rhs"  using prems Inv_ES  
-        by (simp add:Inv_def ardenable_def)
-      with nonempty_yrhs 
-      have "rhs_nonempty (rhs_subst rhs Y (arden_variate Y yrhs))"
-        by (simp add:nonempty_yrhs 
-               rhs_subst_keeps_nonempty arden_variate_keeps_nonempty)
-    } thus ?thesis by (auto simp add:ardenable_def eqs_subst_def)
-  qed
-  moreover have "valid_eqns (eqs_subst ES Y (arden_variate Y yrhs))"
-  proof-
-    have "Y = L (arden_variate Y yrhs)" 
-      using Y_eq_yrhs Inv_ES finite_yrhs nonempty_yrhs      
-      by (rule_tac arden_variate_keeps_eq, (simp add:rexp_of_empty)+)
-    thus ?thesis using Inv_ES 
-      by (clarsimp simp add:valid_eqns_def 
-              eqs_subst_def rhs_subst_keeps_eq Inv_def finite_rhs_def
-                   simp del:L_rhs.simps)
-  qed
-  moreover have 
-    non_empty_subst: "non_empty (eqs_subst ES Y (arden_variate Y yrhs))"
-    using Inv_ES by (auto simp:Inv_def non_empty_def eqs_subst_def)
-  moreover 
-  have self_subst: "self_contained (eqs_subst ES Y (arden_variate Y yrhs))"
-    using Inv_ES eqs_subst_keeps_self_contained by (simp add:Inv_def)
-  ultimately show ?thesis using Inv_ES by (simp add:Inv_def)
-qed
-
-lemma eqs_subst_card_le: 
-  assumes finite: "finite (ES::(string set \<times> rhs_item set) set)"
-  shows "card (eqs_subst ES Y yrhs) <= card ES"
-proof-
-  def f \<equiv> "\<lambda> x. ((fst x)::string set, rhs_subst (snd x) Y yrhs)"
-  have "eqs_subst ES Y yrhs = f ` ES" 
-    apply (auto simp:eqs_subst_def f_def image_def)
-    by (rule_tac x = "(Ya, yrhsa)" in bexI, simp+)
-  thus ?thesis using finite by (auto intro:card_image_le)
-qed
-
-lemma eqs_subst_cls_remains: 
-  "(X, xrhs) \<in> ES \<Longrightarrow> \<exists> xrhs'. (X, xrhs') \<in> (eqs_subst ES Y yrhs)"
-by (auto simp:eqs_subst_def)
-
-lemma card_noteq_1_has_more:
-  assumes card:"card S \<noteq> 1"
-  and e_in: "e \<in> S"
-  and finite: "finite S"
-  obtains e' where "e' \<in> S \<and> e \<noteq> e'" 
-proof-
-  have "card (S - {e}) > 0"
-  proof -
-    have "card S > 1" using card e_in finite  
-      by (case_tac "card S", auto) 
-    thus ?thesis using finite e_in by auto
-  qed
-  hence "S - {e} \<noteq> {}" using finite by (rule_tac notI, simp)
-  thus "(\<And>e'. e' \<in> S \<and> e \<noteq> e' \<Longrightarrow> thesis) \<Longrightarrow> thesis" by auto
-qed
-
-lemma iteration_step: 
-  assumes Inv_ES: "Inv ES"
-  and    X_in_ES: "(X, xrhs) \<in> ES"
-  and    not_T: "card ES \<noteq> 1"
-  shows "\<exists> ES'. (Inv ES' \<and> (\<exists> xrhs'.(X, xrhs') \<in> ES')) \<and> 
-                (card ES', card ES) \<in> less_than" (is "\<exists> ES'. ?P ES'")
-proof -
-  have finite_ES: "finite ES" using Inv_ES by (simp add:Inv_def)
-  then obtain Y yrhs 
-    where Y_in_ES: "(Y, yrhs) \<in> ES" and not_eq: "(X, xrhs) \<noteq> (Y, yrhs)" 
-    using not_T X_in_ES by (drule_tac card_noteq_1_has_more, auto)
-  def ES' == "ES - {(Y, yrhs)}"
-  let ?ES'' = "eqs_subst ES' Y (arden_variate Y yrhs)"
-  have "?P ?ES''"
-  proof -
-    have "Inv ?ES''" using Y_in_ES Inv_ES
-      by (rule_tac eqs_subst_satisfy_Inv, simp add:ES'_def insert_absorb)
-    moreover have "\<exists>xrhs'. (X, xrhs') \<in> ?ES''"  using not_eq X_in_ES
-      by (rule_tac ES = ES' in eqs_subst_cls_remains, auto simp add:ES'_def)
-    moreover have "(card ?ES'', card ES) \<in> less_than" 
-    proof -
-      have "finite ES'" using finite_ES ES'_def by auto
-      moreover have "card ES' < card ES" using finite_ES Y_in_ES
-        by (auto simp:ES'_def card_gt_0_iff intro:diff_Suc_less)
-      ultimately show ?thesis 
-        by (auto dest:eqs_subst_card_le elim:le_less_trans)
-    qed
-    ultimately show ?thesis by simp
-  qed
-  thus ?thesis by blast
-qed
-
-text {*
-  From this point until @{text "hard_direction"}, the hard direction is proved
-  through a simple application of the iteration principle.
-*}
-
-lemma iteration_conc: 
-  assumes history: "Inv ES"
-  and    X_in_ES: "\<exists> xrhs. (X, xrhs) \<in> ES"
-  shows 
-  "\<exists> ES'. (Inv ES' \<and> (\<exists> xrhs'. (X, xrhs') \<in> ES')) \<and> card ES' = 1" 
-                                                          (is "\<exists> ES'. ?P ES'")
-proof (cases "card ES = 1")
-  case True
-  thus ?thesis using history X_in_ES
-    by blast
-next
-  case False  
-  thus ?thesis using history iteration_step X_in_ES
-    by (rule_tac f = card in wf_iter, auto)
-qed
-  
-lemma last_cl_exists_rexp:
-  assumes ES_single: "ES = {(X, xrhs)}" 
-  and Inv_ES: "Inv ES"
-  shows "\<exists> (r::rexp). L r = X" (is "\<exists> r. ?P r")
-proof-
-  let ?A = "arden_variate X xrhs"
-  have "?P (rexp_of_lam ?A)"
-  proof -
-    have "L (rexp_of_lam ?A) = L (lam_of ?A)"
-    proof(rule rexp_of_lam_eq_lam_set)
-      show "finite (arden_variate X xrhs)" using Inv_ES ES_single 
-        by (rule_tac arden_variate_keeps_finite, 
-                       auto simp add:Inv_def finite_rhs_def)
-    qed
-    also have "\<dots> = L ?A"
-    proof-
-      have "lam_of ?A = ?A"
-      proof-
-        have "classes_of ?A = {}" using Inv_ES ES_single
-          by (simp add:arden_variate_removes_cl 
-                       self_contained_def Inv_def lefts_of_def) 
-        thus ?thesis 
-          by (auto simp only:lam_of_def classes_of_def, case_tac x, auto)
-      qed
-      thus ?thesis by simp
-    qed
-    also have "\<dots> = X"
-    proof(rule arden_variate_keeps_eq [THEN sym])
-      show "X = L xrhs" using Inv_ES ES_single 
-        by (auto simp only:Inv_def valid_eqns_def)  
-    next
-      from Inv_ES ES_single show "[] \<notin> L (rexp_of xrhs X)"
-        by(simp add:Inv_def ardenable_def rexp_of_empty finite_rhs_def)
-    next
-      from Inv_ES ES_single show "finite xrhs" 
-        by (simp add:Inv_def finite_rhs_def)
-    qed
-    finally show ?thesis by simp
-  qed
-  thus ?thesis by auto
-qed
-   
-lemma every_eqcl_has_reg: 
-  assumes finite_CS: "finite (UNIV // (\<approx>Lang))"
-  and X_in_CS: "X \<in> (UNIV // (\<approx>Lang))"
-  shows "\<exists> (reg::rexp). L reg = X" (is "\<exists> r. ?E r")
-proof -
-  from X_in_CS have "\<exists> xrhs. (X, xrhs) \<in> (eqs (UNIV  // (\<approx>Lang)))"
-    by (auto simp:eqs_def init_rhs_def)
-  then obtain ES xrhs where Inv_ES: "Inv ES" 
-    and X_in_ES: "(X, xrhs) \<in> ES"
-    and card_ES: "card ES = 1"
-    using finite_CS X_in_CS init_ES_satisfy_Inv iteration_conc
-    by blast
-  hence ES_single_equa: "ES = {(X, xrhs)}" 
-    by (auto simp:Inv_def dest!:card_Suc_Diff1 simp:card_eq_0_iff) 
-  thus ?thesis using Inv_ES
-    by (rule last_cl_exists_rexp)
-qed
-
-lemma finals_in_partitions:
-  "finals Lang \<subseteq> (UNIV // (\<approx>Lang))"
-  by (auto simp:finals_def quotient_def)   
-
-theorem hard_direction: 
-  assumes finite_CS: "finite (UNIV // (\<approx>Lang))"
-  shows   "\<exists> (reg::rexp). Lang = L reg"
-proof -
-  have "\<forall> X \<in> (UNIV // (\<approx>Lang)). \<exists> (reg::rexp). X = L reg" 
-    using finite_CS every_eqcl_has_reg by blast
-  then obtain f 
-    where f_prop: "\<forall> X \<in> (UNIV // (\<approx>Lang)). X = L ((f X)::rexp)" 
-    by (auto dest:bchoice)
-  def rs \<equiv> "f ` (finals Lang)"  
-  have "Lang = \<Union> (finals Lang)" using lang_is_union_of_finals by auto
-  also have "\<dots> = L (folds ALT NULL rs)" 
-  proof -
-    have "finite rs"
-    proof -
-      have "finite (finals Lang)" 
-        using finite_CS finals_in_partitions[of "Lang"]   
-        by (erule_tac finite_subset, simp)
-      thus ?thesis using rs_def by auto
-    qed
-    thus ?thesis 
-      using f_prop rs_def finals_in_partitions[of "Lang"] by auto
-  qed
-  finally show ?thesis by blast
-qed 
-
-section {* Direction: @{text "regular language \<Rightarrow>finite partition"} *}
-
-subsection {* The scheme for this direction *}
-
-text {* 
-  The following convenient notation @{text "x \<approx>Lang y"} means:
-  string @{text "x"} and @{text "y"} are equivalent with respect to 
-  language @{text "Lang"}.
-  *}
-
-definition
-  str_eq ("_ \<approx>_ _")
-where
-  "x \<approx>Lang y \<equiv> (x, y) \<in> (\<approx>Lang)"
-
-text {*
-  The very basic scheme to show the finiteness of the partion generated by a language @{text "Lang"}
-  is by attaching tags to every string. The set of tags are carfully choosen to make it finite.
-  If it can be proved that strings with the same tag are equivlent with respect @{text "Lang"},
-  then the partition given rise by @{text "Lang"} must be finite. The reason for this is a lemma 
-  in standard library (@{text "finite_imageD"}), which says: if the image of an injective 
-  function on a set @{text "A"} is finite, then @{text "A"} is finite. It can be shown that
-  the function obtained by llifting @{text "tag"}
-  to the level of equalent classes (i.e. @{text "((op `) tag)"}) is injective 
-  (by lemma @{text "tag_image_injI"}) and the image of this function is finite 
-  (with the help of lemma @{text "finite_tag_imageI"}).
-
-  BUT, I think this argument can be encapsulated by one lemma instead of the current presentation.
-  *}
-
-lemma eq_class_equalI:
-  "\<lbrakk>X \<in> UNIV // \<approx>lang; Y \<in> UNIV // \<approx>lang; x \<in> X; y \<in> Y; x \<approx>lang y\<rbrakk> 
-                         \<Longrightarrow> X = Y"
-by (auto simp:quotient_def str_eq_rel_def str_eq_def)
-
-lemma tag_image_injI:
-  assumes str_inj: "\<And> x y. tag x = tag (y::string) \<Longrightarrow> x \<approx>lang y"
-  shows "inj_on ((op `) tag) (UNIV // \<approx>lang)"
-proof-
-  { fix X Y
-    assume X_in: "X \<in> UNIV // \<approx>lang"
-      and  Y_in: "Y \<in> UNIV // \<approx>lang"
-      and  tag_eq: "tag ` X = tag ` Y"
-    then obtain x y where "x \<in> X" and "y \<in> Y" and "tag x = tag y"
-      unfolding quotient_def Image_def str_eq_rel_def str_eq_def image_def
-      apply simp by blast
-    with X_in Y_in str_inj
-    have "X = Y" by (rule_tac eq_class_equalI, simp+)
-  }
-  thus ?thesis unfolding inj_on_def by auto
-qed
-
-lemma finite_tag_imageI: 
-  "finite (range tag) \<Longrightarrow> finite (((op `) tag) ` S)"
-apply (rule_tac B = "Pow (tag ` UNIV)" in finite_subset)
-by (auto simp add:image_def Pow_def)
-
-
-subsection {* A small theory for list difference *}
-
-text {*
-  The notion of list diffrence is need to make proofs more readable.
-  *}
-
-(* list_diff:: list substract, once different return tailer *)
-fun list_diff :: "'a list \<Rightarrow> 'a list \<Rightarrow> 'a list" (infix "-" 51)
-where
-  "list_diff []  xs = []" |
-  "list_diff (x#xs) [] = x#xs" |
-  "list_diff (x#xs) (y#ys) = (if x = y then list_diff xs ys else (x#xs))"
-
-lemma [simp]: "(x @ y) - x = y"
-apply (induct x)
-by (case_tac y, simp+)
-
-lemma [simp]: "x - x = []"
-by (induct x, auto)
-
-lemma [simp]: "x = xa @ y \<Longrightarrow> x - xa = y "
-by (induct x, auto)
-
-lemma [simp]: "x - [] = x"
-by (induct x, auto)
-
-lemma [simp]: "(x - y = []) \<Longrightarrow> (x \<le> y)"
-proof-   
-  have "\<exists>xa. x = xa @ (x - y) \<and> xa \<le> y"
-    apply (rule list_diff.induct[of _ x y], simp+)
-    by (clarsimp, rule_tac x = "y # xa" in exI, simp+)
-  thus "(x - y = []) \<Longrightarrow> (x \<le> y)" by simp
-qed
-
-lemma diff_prefix:
-  "\<lbrakk>c \<le> a - b; b \<le> a\<rbrakk> \<Longrightarrow> b @ c \<le> a"
-by (auto elim:prefixE)
-
-lemma diff_diff_appd: 
-  "\<lbrakk>c < a - b; b < a\<rbrakk> \<Longrightarrow> (a - b) - c = a - (b @ c)"
-apply (clarsimp simp:strict_prefix_def)
-by (drule diff_prefix, auto elim:prefixE)
-
-lemma app_eq_cases[rule_format]:
-  "\<forall> x . x @ y = m @ n \<longrightarrow> (x \<le> m \<or> m \<le> x)"
-apply (induct y, simp)
-apply (clarify, drule_tac x = "x @ [a]" in spec)
-by (clarsimp, auto simp:prefix_def)
-
-lemma app_eq_dest:
-  "x @ y = m @ n \<Longrightarrow> 
-               (x \<le> m \<and> (m - x) @ n = y) \<or> (m \<le> x \<and> (x - m) @ y = n)"
-by (frule_tac app_eq_cases, auto elim:prefixE)
-
-subsection {* Lemmas for basic cases *}
-
-text {*
-  The the final result of this direction is in @{text "easier_direction"}, which
-  is an induction on the structure of regular expressions. There is one case 
-  for each regular expression operator. For basic operators such as @{text "NULL, EMPTY, CHAR c"},
-  the finiteness of their language partition can be established directly with no need
-  of taggiing. This section contains several technical lemma for these base cases.
-
-  The inductive cases involve operators @{text "ALT, SEQ"} and @{text "STAR"}. 
-  Tagging functions need to be defined individually for each of them. There will be one
-  dedicated section for each of these cases, and each section goes virtually the same way:
-  gives definition of the tagging function and prove that strings 
-  with the same tag are equivalent.
-  *}
-
-lemma quot_empty_subset:
-  "UNIV // (\<approx>{[]}) \<subseteq> {{[]}, UNIV - {[]}}"
-proof
-  fix x
-  assume "x \<in> UNIV // \<approx>{[]}"
-  then obtain y where h: "x = {z. (y, z) \<in> \<approx>{[]}}" 
-    unfolding quotient_def Image_def by blast
-  show "x \<in> {{[]}, UNIV - {[]}}" 
-  proof (cases "y = []")
-    case True with h
-    have "x = {[]}" by (auto simp:str_eq_rel_def)
-    thus ?thesis by simp
-  next
-    case False with h
-    have "x = UNIV - {[]}" by (auto simp:str_eq_rel_def)
-    thus ?thesis by simp
-  qed
-qed
-
-lemma quot_char_subset:
-  "UNIV // (\<approx>{[c]}) \<subseteq> {{[]},{[c]}, UNIV - {[], [c]}}"
-proof 
-  fix x 
-  assume "x \<in> UNIV // \<approx>{[c]}"
-  then obtain y where h: "x = {z. (y, z) \<in> \<approx>{[c]}}" 
-    unfolding quotient_def Image_def by blast
-  show "x \<in> {{[]},{[c]}, UNIV - {[], [c]}}"
-  proof -
-    { assume "y = []" hence "x = {[]}" using h 
-        by (auto simp:str_eq_rel_def)
-    } moreover {
-      assume "y = [c]" hence "x = {[c]}" using h 
-        by (auto dest!:spec[where x = "[]"] simp:str_eq_rel_def)
-    } moreover {
-      assume "y \<noteq> []" and "y \<noteq> [c]"
-      hence "\<forall> z. (y @ z) \<noteq> [c]" by (case_tac y, auto)
-      moreover have "\<And> p. (p \<noteq> [] \<and> p \<noteq> [c]) = (\<forall> q. p @ q \<noteq> [c])" 
-        by (case_tac p, auto)
-      ultimately have "x = UNIV - {[],[c]}" using h
-        by (auto simp add:str_eq_rel_def)
-    } ultimately show ?thesis by blast
-  qed
-qed
-
-subsection {* The case for @{text "SEQ"}*}
-
-definition 
-  "tag_str_SEQ L\<^isub>1 L\<^isub>2 x \<equiv> 
-       ((\<approx>L\<^isub>1) `` {x}, {(\<approx>L\<^isub>2) `` {x - xa}| xa.  xa \<le> x \<and> xa \<in> L\<^isub>1})"
-
-lemma tag_str_seq_range_finite:
-  "\<lbrakk>finite (UNIV // \<approx>L\<^isub>1); finite (UNIV // \<approx>L\<^isub>2)\<rbrakk> 
-                              \<Longrightarrow> finite (range (tag_str_SEQ L\<^isub>1 L\<^isub>2))"
-apply (rule_tac B = "(UNIV // \<approx>L\<^isub>1) \<times> (Pow (UNIV // \<approx>L\<^isub>2))" in finite_subset)
-by (auto simp:tag_str_SEQ_def Image_def quotient_def split:if_splits)
-
-lemma append_seq_elim:
-  assumes "x @ y \<in> L\<^isub>1 ;; L\<^isub>2"
-  shows "(\<exists> xa \<le> x. xa \<in> L\<^isub>1 \<and> (x - xa) @ y \<in> L\<^isub>2) \<or> 
-          (\<exists> ya \<le> y. (x @ ya) \<in> L\<^isub>1 \<and> (y - ya) \<in> L\<^isub>2)"
-proof-
-  from assms obtain s\<^isub>1 s\<^isub>2 
-    where "x @ y = s\<^isub>1 @ s\<^isub>2" 
-    and in_seq: "s\<^isub>1 \<in> L\<^isub>1 \<and> s\<^isub>2 \<in> L\<^isub>2" 
-    by (auto simp:Seq_def)
-  hence "(x \<le> s\<^isub>1 \<and> (s\<^isub>1 - x) @ s\<^isub>2 = y) \<or> (s\<^isub>1 \<le> x \<and> (x - s\<^isub>1) @ y = s\<^isub>2)"
-    using app_eq_dest by auto
-  moreover have "\<lbrakk>x \<le> s\<^isub>1; (s\<^isub>1 - x) @ s\<^isub>2 = y\<rbrakk> \<Longrightarrow> 
-                       \<exists> ya \<le> y. (x @ ya) \<in> L\<^isub>1 \<and> (y - ya) \<in> L\<^isub>2" 
-    using in_seq by (rule_tac x = "s\<^isub>1 - x" in exI, auto elim:prefixE)
-  moreover have "\<lbrakk>s\<^isub>1 \<le> x; (x - s\<^isub>1) @ y = s\<^isub>2\<rbrakk> \<Longrightarrow> 
-                    \<exists> xa \<le> x. xa \<in> L\<^isub>1 \<and> (x - xa) @ y \<in> L\<^isub>2" 
-    using in_seq by (rule_tac x = s\<^isub>1 in exI, auto)
-  ultimately show ?thesis by blast
-qed
-
-lemma tag_str_SEQ_injI:
-  "tag_str_SEQ L\<^isub>1 L\<^isub>2 m = tag_str_SEQ L\<^isub>1 L\<^isub>2 n \<Longrightarrow> m \<approx>(L\<^isub>1 ;; L\<^isub>2) n"
-proof-
-  { fix x y z
-    assume xz_in_seq: "x @ z \<in> L\<^isub>1 ;; L\<^isub>2"
-    and tag_xy: "tag_str_SEQ L\<^isub>1 L\<^isub>2 x = tag_str_SEQ L\<^isub>1 L\<^isub>2 y"
-    have"y @ z \<in> L\<^isub>1 ;; L\<^isub>2" 
-    proof-
-      have "(\<exists> xa \<le> x. xa \<in> L\<^isub>1 \<and> (x - xa) @ z \<in> L\<^isub>2) \<or> 
-               (\<exists> za \<le> z. (x @ za) \<in> L\<^isub>1 \<and> (z - za) \<in> L\<^isub>2)"
-        using xz_in_seq append_seq_elim by simp
-      moreover {
-        fix xa
-        assume h1: "xa \<le> x" and h2: "xa \<in> L\<^isub>1" and h3: "(x - xa) @ z \<in> L\<^isub>2"
-        obtain ya where "ya \<le> y" and "ya \<in> L\<^isub>1" and "(y - ya) @ z \<in> L\<^isub>2" 
-        proof -
-          have "\<exists> ya.  ya \<le> y \<and> ya \<in> L\<^isub>1 \<and> (x - xa) \<approx>L\<^isub>2 (y - ya)"
-          proof -
-            have "{\<approx>L\<^isub>2 `` {x - xa} |xa. xa \<le> x \<and> xa \<in> L\<^isub>1} = 
-                  {\<approx>L\<^isub>2 `` {y - xa} |xa. xa \<le> y \<and> xa \<in> L\<^isub>1}" 
-                          (is "?Left = ?Right") 
-              using h1 tag_xy by (auto simp:tag_str_SEQ_def)
-            moreover have "\<approx>L\<^isub>2 `` {x - xa} \<in> ?Left" using h1 h2 by auto
-            ultimately have "\<approx>L\<^isub>2 `` {x - xa} \<in> ?Right" by simp
-            thus ?thesis by (auto simp:Image_def str_eq_rel_def str_eq_def)
-          qed
-          with prems show ?thesis by (auto simp:str_eq_rel_def str_eq_def)
-        qed
-        hence "y @ z \<in> L\<^isub>1 ;; L\<^isub>2" by (erule_tac prefixE, auto simp:Seq_def)          
-      } moreover {
-        fix za
-        assume h1: "za \<le> z" and h2: "(x @ za) \<in> L\<^isub>1" and h3: "z - za \<in> L\<^isub>2"
-        hence "y @ za \<in> L\<^isub>1"
-        proof-
-          have "\<approx>L\<^isub>1 `` {x} = \<approx>L\<^isub>1 `` {y}" 
-            using h1 tag_xy by (auto simp:tag_str_SEQ_def)
-          with h2 show ?thesis 
-            by (auto simp:Image_def str_eq_rel_def str_eq_def) 
-        qed
-        with h1 h3 have "y @ z \<in> L\<^isub>1 ;; L\<^isub>2" 
-          by (drule_tac A = L\<^isub>1 in seq_intro, auto elim:prefixE)
-      }
-      ultimately show ?thesis by blast
-    qed
-  } thus "tag_str_SEQ L\<^isub>1 L\<^isub>2 m = tag_str_SEQ L\<^isub>1 L\<^isub>2 n \<Longrightarrow> m \<approx>(L\<^isub>1 ;; L\<^isub>2) n" 
-    by (auto simp add: str_eq_def str_eq_rel_def)
-qed 
-
-lemma quot_seq_finiteI:
-  assumes finite1: "finite (UNIV // \<approx>(L\<^isub>1::string set))"
-  and finite2: "finite (UNIV // \<approx>L\<^isub>2)"
-  shows "finite (UNIV // \<approx>(L\<^isub>1 ;; L\<^isub>2))"
-proof(rule_tac f = "(op `) (tag_str_SEQ L\<^isub>1 L\<^isub>2)" in finite_imageD)
-  show "finite (op ` (tag_str_SEQ L\<^isub>1 L\<^isub>2) ` UNIV // \<approx>L\<^isub>1 ;; L\<^isub>2)" 
-    using finite1 finite2
-    by (auto intro:finite_tag_imageI tag_str_seq_range_finite)
-next
-  show  "inj_on (op ` (tag_str_SEQ L\<^isub>1 L\<^isub>2)) (UNIV // \<approx>L\<^isub>1 ;; L\<^isub>2)"
-    apply (rule tag_image_injI)
-    apply (rule tag_str_SEQ_injI)
-    by (auto intro:tag_image_injI tag_str_SEQ_injI simp:)
-qed
-
-subsection {* The case for @{text "ALT"} *}
-
-definition 
-  "tag_str_ALT L\<^isub>1 L\<^isub>2 (x::string) \<equiv> ((\<approx>L\<^isub>1) `` {x}, (\<approx>L\<^isub>2) `` {x})"
-
-lemma tag_str_alt_range_finite:
-  "\<lbrakk>finite (UNIV // \<approx>L\<^isub>1); finite (UNIV // \<approx>L\<^isub>2)\<rbrakk> 
-                              \<Longrightarrow> finite (range (tag_str_ALT L\<^isub>1 L\<^isub>2))"
-apply (rule_tac B = "(UNIV // \<approx>L\<^isub>1) \<times> (UNIV // \<approx>L\<^isub>2)" in finite_subset)
-by (auto simp:tag_str_ALT_def Image_def quotient_def)
-
-lemma quot_union_finiteI:
-  assumes finite1: "finite (UNIV // \<approx>(L\<^isub>1::string set))"
-  and finite2: "finite (UNIV // \<approx>L\<^isub>2)"
-  shows "finite (UNIV // \<approx>(L\<^isub>1 \<union> L\<^isub>2))"
-proof(rule_tac f = "(op `) (tag_str_ALT L\<^isub>1 L\<^isub>2)" in finite_imageD)
-  show "finite (op ` (tag_str_ALT L\<^isub>1 L\<^isub>2) ` UNIV // \<approx>L\<^isub>1 \<union> L\<^isub>2)" 
-    using finite1 finite2
-    by (auto intro:finite_tag_imageI tag_str_alt_range_finite)
-next
-  show "inj_on (op ` (tag_str_ALT L\<^isub>1 L\<^isub>2)) (UNIV // \<approx>L\<^isub>1 \<union> L\<^isub>2)"
-  proof-
-    have "\<And>m n. tag_str_ALT L\<^isub>1 L\<^isub>2 m = tag_str_ALT L\<^isub>1 L\<^isub>2 n 
-                         \<Longrightarrow> m \<approx>(L\<^isub>1 \<union> L\<^isub>2) n"
-      unfolding tag_str_ALT_def str_eq_def Image_def str_eq_rel_def by auto
-    thus ?thesis by (auto intro:tag_image_injI)
-  qed
-qed
-
-
-subsection {*
-  The case for @{text "STAR"}
-  *}
-
-text {* 
-  This turned out to be the most tricky case. 
-  *} (* I will make some illustrations for it. *)
-
-definition 
-  "tag_str_STAR L\<^isub>1 x \<equiv> {(\<approx>L\<^isub>1) `` {x - xa} | xa. xa < x \<and> xa \<in> L\<^isub>1\<star>}"
-
-lemma finite_set_has_max: "\<lbrakk>finite A; A \<noteq> {}\<rbrakk> \<Longrightarrow> 
-           (\<exists> max \<in> A. \<forall> a \<in> A. f a <= (f max :: nat))"
-proof (induct rule:finite.induct)
-  case emptyI thus ?case by simp
-next
-  case (insertI A a)
-  show ?case
-  proof (cases "A = {}")
-    case True thus ?thesis by (rule_tac x = a in bexI, auto)
-  next
-    case False
-    with prems obtain max 
-      where h1: "max \<in> A" 
-      and h2: "\<forall>a\<in>A. f a \<le> f max" by blast
-    show ?thesis
-    proof (cases "f a \<le> f max")
-      assume "f a \<le> f max"
-      with h1 h2 show ?thesis by (rule_tac x = max in bexI, auto)
-    next
-      assume "\<not> (f a \<le> f max)"
-      thus ?thesis using h2 by (rule_tac x = a in bexI, auto)
-    qed
-  qed
-qed
-
-lemma finite_strict_prefix_set: "finite {xa. xa < (x::string)}"
-apply (induct x rule:rev_induct, simp)
-apply (subgoal_tac "{xa. xa < xs @ [x]} = {xa. xa < xs} \<union> {xs}")
-by (auto simp:strict_prefix_def)
-
-
-lemma tag_str_star_range_finite:
-  "finite (UNIV // \<approx>L\<^isub>1) \<Longrightarrow> finite (range (tag_str_STAR L\<^isub>1))"
-apply (rule_tac B = "Pow (UNIV // \<approx>L\<^isub>1)" in finite_subset)
-by (auto simp:tag_str_STAR_def Image_def 
-                       quotient_def split:if_splits)
-
-lemma tag_str_STAR_injI:
-  "tag_str_STAR L\<^isub>1 m = tag_str_STAR L\<^isub>1 n \<Longrightarrow> m \<approx>(L\<^isub>1\<star>) n"
-proof-
-  { fix x y z
-    assume xz_in_star: "x @ z \<in> L\<^isub>1\<star>"
-    and tag_xy: "tag_str_STAR L\<^isub>1 x = tag_str_STAR L\<^isub>1 y"
-    have "y @ z \<in> L\<^isub>1\<star>"
-    proof(cases "x = []")
-      case True
-      with tag_xy have "y = []" 
-        by (auto simp:tag_str_STAR_def strict_prefix_def)
-      thus ?thesis using xz_in_star True by simp
-    next
-      case False
-      obtain x_max 
-        where h1: "x_max < x" 
-        and h2: "x_max \<in> L\<^isub>1\<star>" 
-        and h3: "(x - x_max) @ z \<in> L\<^isub>1\<star>" 
-        and h4:"\<forall> xa < x. xa \<in> L\<^isub>1\<star> \<and> (x - xa) @ z \<in> L\<^isub>1\<star> 
-                                     \<longrightarrow> length xa \<le> length x_max"
-      proof-
-        let ?S = "{xa. xa < x \<and> xa \<in> L\<^isub>1\<star> \<and> (x - xa) @ z \<in> L\<^isub>1\<star>}"
-        have "finite ?S"
-          by (rule_tac B = "{xa. xa < x}" in finite_subset, 
-                                auto simp:finite_strict_prefix_set)
-        moreover have "?S \<noteq> {}" using False xz_in_star
-          by (simp, rule_tac x = "[]" in exI, auto simp:strict_prefix_def)
-        ultimately have "\<exists> max \<in> ?S. \<forall> a \<in> ?S. length a \<le> length max" 
-          using finite_set_has_max by blast
-        with prems show ?thesis by blast
-      qed
-      obtain ya 
-        where h5: "ya < y" and h6: "ya \<in> L\<^isub>1\<star>" and h7: "(x - x_max) \<approx>L\<^isub>1 (y - ya)"
-      proof-
-        from tag_xy have "{\<approx>L\<^isub>1 `` {x - xa} |xa. xa < x \<and> xa \<in> L\<^isub>1\<star>} = 
-          {\<approx>L\<^isub>1 `` {y - xa} |xa. xa < y \<and> xa \<in> L\<^isub>1\<star>}" (is "?left = ?right")
-          by (auto simp:tag_str_STAR_def)
-        moreover have "\<approx>L\<^isub>1 `` {x - x_max} \<in> ?left" using h1 h2 by auto
-        ultimately have "\<approx>L\<^isub>1 `` {x - x_max} \<in> ?right" by simp
-        with prems show ?thesis apply 
-          (simp add:Image_def str_eq_rel_def str_eq_def) by blast
-      qed      
-      have "(y - ya) @ z \<in> L\<^isub>1\<star>" 
-      proof-
-        from h3 h1 obtain a b where a_in: "a \<in> L\<^isub>1" 
-          and a_neq: "a \<noteq> []" and b_in: "b \<in> L\<^isub>1\<star>" 
-          and ab_max: "(x - x_max) @ z = a @ b" 
-          by (drule_tac star_decom, auto simp:strict_prefix_def elim:prefixE)
-        have "(x - x_max) \<le> a \<and> (a - (x - x_max)) @ b = z" 
-        proof -
-          have "((x - x_max) \<le> a \<and> (a - (x - x_max)) @ b = z) \<or> 
-                            (a < (x - x_max) \<and> ((x - x_max) - a) @ z = b)" 
-            using app_eq_dest[OF ab_max] by (auto simp:strict_prefix_def)
-          moreover { 
-            assume np: "a < (x - x_max)" and b_eqs: " ((x - x_max) - a) @ z = b"
-            have "False"
-            proof -
-              let ?x_max' = "x_max @ a"
-              have "?x_max' < x" 
-                using np h1 by (clarsimp simp:strict_prefix_def diff_prefix) 
-              moreover have "?x_max' \<in> L\<^isub>1\<star>" 
-                using a_in h2 by (simp add:star_intro3) 
-              moreover have "(x - ?x_max') @ z \<in> L\<^isub>1\<star>" 
-                using b_eqs b_in np h1 by (simp add:diff_diff_appd)
-              moreover have "\<not> (length ?x_max' \<le> length x_max)" 
-                using a_neq by simp
-              ultimately show ?thesis using h4 by blast
-            qed 
-          } ultimately show ?thesis by blast
-        qed
-        then obtain za where z_decom: "z = za @ b" 
-          and x_za: "(x - x_max) @ za \<in> L\<^isub>1" 
-          using a_in by (auto elim:prefixE)        
-        from x_za h7 have "(y - ya) @ za \<in> L\<^isub>1" 
-          by (auto simp:str_eq_def str_eq_rel_def)
-        with z_decom b_in show ?thesis by (auto dest!:step[of "(y - ya) @ za"])
-      qed
-      with h5 h6 show ?thesis 
-        by (drule_tac star_intro1, auto simp:strict_prefix_def elim:prefixE)
-    qed      
-  } thus "tag_str_STAR L\<^isub>1 m = tag_str_STAR L\<^isub>1 n \<Longrightarrow> m \<approx>(L\<^isub>1\<star>) n"
-    by (auto simp add:str_eq_def str_eq_rel_def)
-qed
-
-lemma quot_star_finiteI:
-  assumes finite: "finite (UNIV // \<approx>(L\<^isub>1::string set))"
-  shows "finite (UNIV // \<approx>(L\<^isub>1\<star>))"
-proof(rule_tac f = "(op `) (tag_str_STAR L\<^isub>1)" in finite_imageD)
-  show "finite (op ` (tag_str_STAR L\<^isub>1) ` UNIV // \<approx>L\<^isub>1\<star>)" using finite
-    by (auto intro:finite_tag_imageI tag_str_star_range_finite)
-next
-  show "inj_on (op ` (tag_str_STAR L\<^isub>1)) (UNIV // \<approx>L\<^isub>1\<star>)"
-    by (auto intro:tag_image_injI tag_str_STAR_injI)
-qed
-
-subsection {*
-  The main lemma
-  *}
-
-lemma easier_directio\<nu>:
-  "Lang = L (r::rexp) \<Longrightarrow> finite (UNIV // (\<approx>Lang))"
-proof (induct arbitrary:Lang rule:rexp.induct)
-  case NULL
-  have "UNIV // (\<approx>{}) \<subseteq> {UNIV} "
-    by (auto simp:quotient_def str_eq_rel_def str_eq_def)
-  with prems show "?case" by (auto intro:finite_subset)
-next
-  case EMPTY
-  have "UNIV // (\<approx>{[]}) \<subseteq> {{[]}, UNIV - {[]}}" 
-    by (rule quot_empty_subset)
-  with prems show ?case by (auto intro:finite_subset)
-next
-  case (CHAR c)
-  have "UNIV // (\<approx>{[c]}) \<subseteq> {{[]},{[c]}, UNIV - {[], [c]}}" 
-    by (rule quot_char_subset)
-  with prems show ?case by (auto intro:finite_subset)
-next
-  case (SEQ r\<^isub>1 r\<^isub>2)
-  have "\<lbrakk>finite (UNIV // \<approx>(L r\<^isub>1)); finite (UNIV // \<approx>(L r\<^isub>2))\<rbrakk> 
-            \<Longrightarrow> finite (UNIV // \<approx>(L r\<^isub>1 ;; L r\<^isub>2))"
-    by (erule quot_seq_finiteI, simp)
-  with prems show ?case by simp
-next
-  case (ALT r\<^isub>1 r\<^isub>2)
-  have "\<lbrakk>finite (UNIV // \<approx>(L r\<^isub>1)); finite (UNIV // \<approx>(L r\<^isub>2))\<rbrakk> 
-            \<Longrightarrow> finite (UNIV // \<approx>(L r\<^isub>1 \<union> L r\<^isub>2))"
-    by (erule quot_union_finiteI, simp)
-  with prems show ?case by simp  
-next
-  case (STAR r)
-  have "finite (UNIV // \<approx>(L r)) 
-            \<Longrightarrow> finite (UNIV // \<approx>((L r)\<star>))"
-    by (erule quot_star_finiteI)
-  with prems show ?case by simp
-qed 
-
-end
-
+theory Myhill
+  imports Main List_Prefix Prefix_subtract
+begin
+
+section {* Preliminary definitions *}
+
+text {* Sequential composition of two languages @{text "L1"} and @{text "L2"} *}
+definition Seq :: "string set \<Rightarrow> string set \<Rightarrow> string set" ("_ ;; _" [100,100] 100)
+where 
+  "L1 ;; L2 = {s1 @ s2 | s1 s2. s1 \<in> L1 \<and> s2 \<in> L2}"
+
+text {* Transitive closure of language @{text "L"}. *}
+inductive_set
+  Star :: "string set \<Rightarrow> string set" ("_\<star>" [101] 102)
+  for L :: "string set"
+where
+  start[intro]: "[] \<in> L\<star>"
+| step[intro]:  "\<lbrakk>s1 \<in> L; s2 \<in> L\<star>\<rbrakk> \<Longrightarrow> s1@s2 \<in> L\<star>" 
+
+text {* Some properties of operator @{text ";;"}.*}
+
+lemma seq_union_distrib:
+  "(A \<union> B) ;; C = (A ;; C) \<union> (B ;; C)"
+by (auto simp:Seq_def)
+
+lemma seq_intro:
+  "\<lbrakk>x \<in> A; y \<in> B\<rbrakk> \<Longrightarrow> x @ y \<in> A ;; B "
+by (auto simp:Seq_def)
+
+lemma seq_assoc:
+  "(A ;; B) ;; C = A ;; (B ;; C)"
+apply(auto simp:Seq_def)
+apply blast
+by (metis append_assoc)
+
+lemma star_intro1[rule_format]: "x \<in> lang\<star> \<Longrightarrow> \<forall> y. y \<in> lang\<star> \<longrightarrow> x @ y \<in> lang\<star>"
+by (erule Star.induct, auto)
+
+lemma star_intro2: "y \<in> lang \<Longrightarrow> y \<in> lang\<star>"
+by (drule step[of y lang "[]"], auto simp:start)
+
+lemma star_intro3[rule_format]: 
+  "x \<in> lang\<star> \<Longrightarrow> \<forall>y . y \<in> lang \<longrightarrow> x @ y \<in> lang\<star>"
+by (erule Star.induct, auto intro:star_intro2)
+
+lemma star_decom: 
+  "\<lbrakk>x \<in> lang\<star>; x \<noteq> []\<rbrakk> \<Longrightarrow>(\<exists> a b. x = a @ b \<and> a \<noteq> [] \<and> a \<in> lang \<and> b \<in> lang\<star>)"
+by (induct x rule: Star.induct, simp, blast)
+
+lemma star_decom': 
+  "\<lbrakk>x \<in> lang\<star>; x \<noteq> []\<rbrakk> \<Longrightarrow> \<exists>a b. x = a @ b \<and> a \<in> lang\<star> \<and> b \<in> lang"
+apply (induct x rule:Star.induct, simp)
+apply (case_tac "s2 = []")
+apply (rule_tac x = "[]" in exI, rule_tac x = s1 in exI, simp add:start)
+apply (simp, (erule exE| erule conjE)+)
+by (rule_tac x = "s1 @ a" in exI, rule_tac x = b in exI, simp add:step)
+
+text {* Ardens lemma expressed at the level of language, rather than the level of regular expression. *}
+
+theorem ardens_revised:
+  assumes nemp: "[] \<notin> A"
+  shows "(X = X ;; A \<union> B) \<longleftrightarrow> (X = B ;; A\<star>)"
+proof
+  assume eq: "X = B ;; A\<star>"
+  have "A\<star> =  {[]} \<union> A\<star> ;; A" 
+    by (auto simp:Seq_def star_intro3 star_decom')  
+  then have "B ;; A\<star> = B ;; ({[]} \<union> A\<star> ;; A)" 
+    unfolding Seq_def by simp
+  also have "\<dots> = B \<union> B ;; (A\<star> ;; A)"  
+    unfolding Seq_def by auto
+  also have "\<dots> = B \<union> (B ;; A\<star>) ;; A" 
+    by (simp only:seq_assoc)
+  finally show "X = X ;; A \<union> B" 
+    using eq by blast 
+next
+  assume eq': "X = X ;; A \<union> B"
+  hence c1': "\<And> x. x \<in> B \<Longrightarrow> x \<in> X" 
+    and c2': "\<And> x y. \<lbrakk>x \<in> X; y \<in> A\<rbrakk> \<Longrightarrow> x @ y \<in> X" 
+    using Seq_def by auto
+  show "X = B ;; A\<star>" 
+  proof
+    show "B ;; A\<star> \<subseteq> X"
+    proof-
+      { fix x y
+        have "\<lbrakk>y \<in> A\<star>; x \<in> X\<rbrakk> \<Longrightarrow> x @ y \<in> X "
+          apply (induct arbitrary:x rule:Star.induct, simp)
+          by (auto simp only:append_assoc[THEN sym] dest:c2')
+      } thus ?thesis using c1' by (auto simp:Seq_def) 
+    qed
+  next
+    show "X \<subseteq> B ;; A\<star>"
+    proof-
+      { fix x 
+        have "x \<in> X \<Longrightarrow> x \<in> B ;; A\<star>"
+        proof (induct x taking:length rule:measure_induct)
+          fix z
+          assume hyps: 
+            "\<forall>y. length y < length z \<longrightarrow> y \<in> X \<longrightarrow> y \<in> B ;; A\<star>" 
+            and z_in: "z \<in> X"
+          show "z \<in> B ;; A\<star>"
+          proof (cases "z \<in> B")
+            case True thus ?thesis by (auto simp:Seq_def start)
+          next
+            case False hence "z \<in> X ;; A" using eq' z_in by auto
+            then obtain za zb where za_in: "za \<in> X" 
+              and zab: "z = za @ zb \<and> zb \<in> A" and zbne: "zb \<noteq> []" 
+              using nemp unfolding Seq_def by blast
+            from zbne zab have "length za < length z" by auto
+            with za_in hyps have "za \<in> B ;; A\<star>" by blast
+            hence "za @ zb \<in> B ;; A\<star>" using zab 
+              by (clarsimp simp:Seq_def, blast dest:star_intro3)
+            thus ?thesis using zab by simp       
+          qed
+        qed 
+      } thus ?thesis by blast
+    qed
+  qed
+qed
+
+
+text {* The syntax of regular expressions is defined by the datatype @{text "rexp"}. *}
+datatype rexp =
+  NULL
+| EMPTY
+| CHAR char
+| SEQ rexp rexp
+| ALT rexp rexp
+| STAR rexp
+
+
+text {* 
+  The following @{text "L"} is an overloaded operator, where @{text "L(x)"} evaluates to 
+  the language represented by the syntactic object @{text "x"}.
+*}
+consts L:: "'a \<Rightarrow> string set"
+
+
+text {* 
+  The @{text "L(rexp)"} for regular expression @{text "rexp"} is defined by the 
+  following overloading function @{text "L_rexp"}.
+*}
+overloading L_rexp \<equiv> "L::  rexp \<Rightarrow> string set"
+begin
+fun
+  L_rexp :: "rexp \<Rightarrow> string set"
+where
+    "L_rexp (NULL) = {}"
+  | "L_rexp (EMPTY) = {[]}"
+  | "L_rexp (CHAR c) = {[c]}"
+  | "L_rexp (SEQ r1 r2) = (L_rexp r1) ;; (L_rexp r2)"
+  | "L_rexp (ALT r1 r2) = (L_rexp r1) \<union> (L_rexp r2)"
+  | "L_rexp (STAR r) = (L_rexp r)\<star>"
+end
+
+text {*
+  To obtain equational system out of finite set of equivalent classes, a fold operation
+  on finite set @{text "folds"} is defined. The use of @{text "SOME"} makes @{text "fold"}
+  more robust than the @{text "fold"} in Isabelle library. The expression @{text "folds f"}
+  makes sense when @{text "f"} is not @{text "associative"} and @{text "commutitive"},
+  while @{text "fold f"} does not.  
+*}
+
+definition 
+  folds :: "('a \<Rightarrow> 'b \<Rightarrow> 'b) \<Rightarrow> 'b \<Rightarrow> 'a set \<Rightarrow> 'b"
+where
+  "folds f z S \<equiv> SOME x. fold_graph f z S x"
+
+text {* 
+  The following lemma assures that the arbitrary choice made by the @{text "SOME"} in @{text "folds"}
+  does not affect the @{text "L"}-value of the resultant regular expression. 
+  *}
+lemma folds_alt_simp [simp]:
+  "finite rs \<Longrightarrow> L (folds ALT NULL rs) = \<Union> (L ` rs)"
+apply (rule set_ext, simp add:folds_def)
+apply (rule someI2_ex, erule finite_imp_fold_graph)
+by (erule fold_graph.induct, auto)
+
+(* Just a technical lemma. *)
+lemma [simp]:
+  shows "(x, y) \<in> {(x, y). P x y} \<longleftrightarrow> P x y"
+by simp
+
+text {*
+  @{text "\<approx>L"} is an equivalent class defined by language @{text "Lang"}.
+*}
+definition
+  str_eq_rel ("\<approx>_")
+where
+  "\<approx>Lang \<equiv> {(x, y).  (\<forall>z. x @ z \<in> Lang \<longleftrightarrow> y @ z \<in> Lang)}"
+
+text {* 
+  Among equivlant clases of @{text "\<approx>Lang"}, the set @{text "finals(Lang)"} singles out 
+  those which contains strings from @{text "Lang"}.
+*}
+
+definition 
+   "finals Lang \<equiv> {\<approx>Lang `` {x} | x . x \<in> Lang}"
+
+text {* 
+  The following lemma show the relationshipt between @{text "finals(Lang)"} and @{text "Lang"}.
+*}
+lemma lang_is_union_of_finals: 
+  "Lang = \<Union> finals(Lang)"
+proof 
+  show "Lang \<subseteq> \<Union> (finals Lang)"
+  proof
+    fix x
+    assume "x \<in> Lang"   
+    thus "x \<in> \<Union> (finals Lang)"
+      apply (simp add:finals_def, rule_tac x = "(\<approx>Lang) `` {x}" in exI)
+      by (auto simp:Image_def str_eq_rel_def)    
+  qed
+next
+  show "\<Union> (finals Lang) \<subseteq> Lang"
+    apply (clarsimp simp:finals_def str_eq_rel_def)
+    by (drule_tac x = "[]" in spec, auto)
+qed
+
+section {* Direction @{text "finite partition \<Rightarrow> regular language"}*}
+
+text {* 
+  The relationship between equivalent classes can be described by an
+  equational system.
+  For example, in equational system \eqref{example_eqns},  $X_0, X_1$ are equivalent 
+  classes. The first equation says every string in $X_0$ is obtained either by
+  appending one $b$ to a string in $X_0$ or by appending one $a$ to a string in
+  $X_1$ or just be an empty string (represented by the regular expression $\lambda$). Similary,
+  the second equation tells how the strings inside $X_1$ are composed.
+  \begin{equation}\label{example_eqns}
+    \begin{aligned}
+      X_0 & = X_0 b + X_1 a + \lambda \\
+      X_1 & = X_0 a + X_1 b
+    \end{aligned}
+  \end{equation}
+  The summands on the right hand side is represented by the following data type
+  @{text "rhs_item"}, mnemonic for 'right hand side item'.
+  Generally, there are two kinds of right hand side items, one kind corresponds to
+  pure regular expressions, like the $\lambda$ in \eqref{example_eqns}, the other kind corresponds to
+  transitions from one one equivalent class to another, like the $X_0 b, X_1 a$ etc.
+  *}
+
+datatype rhs_item = 
+   Lam "rexp"                           (* Lambda *)
+ | Trn "(string set)" "rexp"              (* Transition *)
+
+text {*
+  In this formalization, pure regular expressions like $\lambda$ is 
+  repsented by @{text "Lam(EMPTY)"}, while transitions like $X_0 a$ is represented by $Trn~X_0~(CHAR~a)$.
+  *}
+
+text {*
+  The functions @{text "the_r"} and @{text "the_Trn"} are used to extract
+  subcomponents from right hand side items.
+  *}
+
+fun the_r :: "rhs_item \<Rightarrow> rexp"
+where "the_r (Lam r) = r"
+
+fun the_Trn:: "rhs_item \<Rightarrow> (string set \<times> rexp)"
+where "the_Trn (Trn Y r) = (Y, r)"
+
+text {*
+  Every right hand side item @{text "itm"} defines a string set given 
+  @{text "L(itm)"}, defined as:
+*}
+overloading L_rhs_e \<equiv> "L:: rhs_item \<Rightarrow> string set"
+begin
+  fun L_rhs_e:: "rhs_item \<Rightarrow> string set"
+  where
+     "L_rhs_e (Lam r) = L r" |
+     "L_rhs_e (Trn X r) = X ;; L r"
+end
+
+text {*
+  The right hand side of every equation is represented by a set of
+  items. The string set defined by such a set @{text "itms"} is given
+  by @{text "L(itms)"}, defined as:
+*}
+
+overloading L_rhs \<equiv> "L:: rhs_item set \<Rightarrow> string set"
+begin
+   fun L_rhs:: "rhs_item set \<Rightarrow> string set"
+   where "L_rhs rhs = \<Union> (L ` rhs)"
+end
+
+text {* 
+  Given a set of equivalent classses @{text "CS"} and one equivalent class @{text "X"} among
+  @{text "CS"}, the term @{text "init_rhs CS X"} is used to extract the right hand side of
+  the equation describing the formation of @{text "X"}. The definition of @{text "init_rhs"}
+  is:
+  *}
+
+definition
+  "init_rhs CS X \<equiv>  
+      if ([] \<in> X) then 
+          {Lam(EMPTY)} \<union> {Trn Y (CHAR c) | Y c. Y \<in> CS \<and> Y ;; {[c]} \<subseteq> X}
+      else 
+          {Trn Y (CHAR c)| Y c. Y \<in> CS \<and> Y ;; {[c]} \<subseteq> X}"
+
+text {*
+  In the definition of @{text "init_rhs"}, the term 
+  @{text "{Trn Y (CHAR c)| Y c. Y \<in> CS \<and> Y ;; {[c]} \<subseteq> X}"} appearing on both branches
+  describes the formation of strings in @{text "X"} out of transitions, while 
+  the term @{text "{Lam(EMPTY)}"} describes the empty string which is intrinsically contained in
+  @{text "X"} rather than by transition. This @{text "{Lam(EMPTY)}"} corresponds to 
+  the $\lambda$ in \eqref{example_eqns}.
+
+  With the help of @{text "init_rhs"}, the equitional system descrbing the formation of every
+  equivalent class inside @{text "CS"} is given by the following @{text "eqs(CS)"}.
+  *}
+
+definition "eqs CS \<equiv> {(X, init_rhs CS X) | X.  X \<in> CS}"
+(************ arden's lemma variation ********************)
+
+text {* 
+  The following @{text "items_of rhs X"} returns all @{text "X"}-items in @{text "rhs"}.
+  *}
+definition
+  "items_of rhs X \<equiv> {Trn X r | r. (Trn X r) \<in> rhs}"
+
+text {* 
+  The following @{text "rexp_of rhs X"} combines all regular expressions in @{text "X"}-items
+  using @{text "ALT"} to form a single regular expression. 
+  It will be used later to implement @{text "arden_variate"} and @{text "rhs_subst"}.
+  *}
+
+definition 
+  "rexp_of rhs X \<equiv> folds ALT NULL ((snd o the_Trn) ` items_of rhs X)"
+
+text {* 
+  The following @{text "lam_of rhs"} returns all pure regular expression items in @{text "rhs"}.
+  *}
+
+definition
+  "lam_of rhs \<equiv> {Lam r | r. Lam r \<in> rhs}"
+
+text {*
+  The following @{text "rexp_of_lam rhs"} combines pure regular expression items in @{text "rhs"}
+  using @{text "ALT"} to form a single regular expression. 
+  When all variables inside @{text "rhs"} are eliminated, @{text "rexp_of_lam rhs"}
+  is used to compute compute the regular expression corresponds to @{text "rhs"}.
+  *}
+
+definition
+  "rexp_of_lam rhs \<equiv> folds ALT NULL (the_r ` lam_of rhs)"
+
+text {*
+  The following @{text "attach_rexp rexp' itm"} attach 
+  the regular expression @{text "rexp'"} to
+  the right of right hand side item @{text "itm"}.
+  *}
+
+fun attach_rexp :: "rexp \<Rightarrow> rhs_item \<Rightarrow> rhs_item"
+where
+  "attach_rexp rexp' (Lam rexp)   = Lam (SEQ rexp rexp')"
+| "attach_rexp rexp' (Trn X rexp) = Trn X (SEQ rexp rexp')"
+
+text {* 
+  The following @{text "append_rhs_rexp rhs rexp"} attaches 
+  @{text "rexp"} to every item in @{text "rhs"}.
+  *}
+
+definition
+  "append_rhs_rexp rhs rexp \<equiv> (attach_rexp rexp) ` rhs"
+
+text {*
+  With the help of the two functions immediately above, Ardens'
+  transformation on right hand side @{text "rhs"} is implemented
+  by the following function @{text "arden_variate X rhs"}.
+  After this transformation, the recursive occurent of @{text "X"}
+  in @{text "rhs"} will be eliminated, while the 
+  string set defined by @{text "rhs"} is kept unchanged.
+  *}
+definition 
+  "arden_variate X rhs \<equiv> 
+        append_rhs_rexp (rhs - items_of rhs X) (STAR (rexp_of rhs X))"
+
+
+(*********** substitution of ES *************)
+
+text {* 
+  Suppose the equation defining @{text "X"} is $X = xrhs$,
+  the purpose of @{text "rhs_subst"} is to substitute all occurences of @{text "X"} in
+  @{text "rhs"} by @{text "xrhs"}.
+  A litte thought may reveal that the final result
+  should be: first append $(a_1 | a_2 | \ldots | a_n)$ to every item of @{text "xrhs"} and then
+  union the result with all non-@{text "X"}-items of @{text "rhs"}.
+ *}
+definition 
+  "rhs_subst rhs X xrhs \<equiv> 
+        (rhs - (items_of rhs X)) \<union> (append_rhs_rexp xrhs (rexp_of rhs X))"
+
+text {*
+  Suppose the equation defining @{text "X"} is $X = xrhs$, the follwing
+  @{text "eqs_subst ES X xrhs"} substitute @{text "xrhs"} into every equation
+  of the equational system @{text "ES"}.
+  *}
+
+definition
+  "eqs_subst ES X xrhs \<equiv> {(Y, rhs_subst yrhs X xrhs) | Y yrhs. (Y, yrhs) \<in> ES}"
+
+text {*
+  The computation of regular expressions for equivalent classes is accomplished
+  using a iteration principle given by the following lemma.
+  *}
+
+lemma wf_iter [rule_format]: 
+  fixes f
+  assumes step: "\<And> e. \<lbrakk>P e; \<not> Q e\<rbrakk> \<Longrightarrow> (\<exists> e'. P e' \<and>  (f(e'), f(e)) \<in> less_than)"
+  shows pe:     "P e \<longrightarrow> (\<exists> e'. P e' \<and>  Q e')"
+proof(induct e rule: wf_induct 
+           [OF wf_inv_image[OF wf_less_than, where f = "f"]], clarify)
+  fix x 
+  assume h [rule_format]: 
+    "\<forall>y. (y, x) \<in> inv_image less_than f \<longrightarrow> P y \<longrightarrow> (\<exists>e'. P e' \<and> Q e')"
+    and px: "P x"
+  show "\<exists>e'. P e' \<and> Q e'"
+  proof(cases "Q x")
+    assume "Q x" with px show ?thesis by blast
+  next
+    assume nq: "\<not> Q x"
+    from step [OF px nq]
+    obtain e' where pe': "P e'" and ltf: "(f e', f x) \<in> less_than" by auto
+    show ?thesis
+    proof(rule h)
+      from ltf show "(e', x) \<in> inv_image less_than f" 
+	by (simp add:inv_image_def)
+    next
+      from pe' show "P e'" .
+    qed
+  qed
+qed
+
+text {*
+  The @{text "P"} in lemma @{text "wf_iter"} is an invaiant kept throughout the iteration procedure.
+  The particular invariant used to solve our problem is defined by function @{text "Inv(ES)"},
+  an invariant over equal system @{text "ES"}.
+  Every definition starting next till @{text "Inv"} stipulates a property to be satisfied by @{text "ES"}.
+*}
+
+text {* 
+  Every variable is defined at most onece in @{text "ES"}.
+  *}
+definition 
+  "distinct_equas ES \<equiv> 
+            \<forall> X rhs rhs'. (X, rhs) \<in> ES \<and> (X, rhs') \<in> ES \<longrightarrow> rhs = rhs'"
+text {* 
+  Every equation in @{text "ES"} (represented by @{text "(X, rhs)"}) is valid, i.e. @{text "(X = L rhs)"}.
+  *}
+definition 
+  "valid_eqns ES \<equiv> \<forall> X rhs. (X, rhs) \<in> ES \<longrightarrow> (X = L rhs)"
+
+text {*
+  The following @{text "rhs_nonempty rhs"} requires regular expressions occuring in transitional 
+  items of @{text "rhs"} does not contain empty string. This is necessary for
+  the application of Arden's transformation to @{text "rhs"}.
+  *}
+definition 
+  "rhs_nonempty rhs \<equiv> (\<forall> Y r. Trn Y r \<in> rhs \<longrightarrow> [] \<notin> L r)"
+
+text {*
+  The following @{text "ardenable ES"} requires that Arden's transformation is applicable
+  to every equation of equational system @{text "ES"}.
+  *}
+definition 
+  "ardenable ES \<equiv> \<forall> X rhs. (X, rhs) \<in> ES \<longrightarrow> rhs_nonempty rhs"
+
+(* The following non_empty seems useless. *)
+definition 
+  "non_empty ES \<equiv> \<forall> X rhs. (X, rhs) \<in> ES \<longrightarrow> X \<noteq> {}"
+
+text {*
+  The following @{text "finite_rhs ES"} requires every equation in @{text "rhs"} be finite.
+  *}
+definition
+  "finite_rhs ES \<equiv> \<forall> X rhs. (X, rhs) \<in> ES \<longrightarrow> finite rhs"
+
+text {*
+  The following @{text "classes_of rhs"} returns all variables (or equivalent classes)
+  occuring in @{text "rhs"}.
+  *}
+definition 
+  "classes_of rhs \<equiv> {X. \<exists> r. Trn X r \<in> rhs}"
+
+text {*
+  The following @{text "lefts_of ES"} returns all variables 
+  defined by equational system @{text "ES"}.
+  *}
+definition
+  "lefts_of ES \<equiv> {Y | Y yrhs. (Y, yrhs) \<in> ES}"
+
+text {*
+  The following @{text "self_contained ES"} requires that every
+  variable occuring on the right hand side of equations is already defined by some
+  equation in @{text "ES"}.
+  *}
+definition 
+  "self_contained ES \<equiv> \<forall> (X, xrhs) \<in> ES. classes_of xrhs \<subseteq> lefts_of ES"
+
+
+text {*
+  The invariant @{text "Inv(ES)"} is a conjunction of all the previously defined constaints.
+  *}
+definition 
+  "Inv ES \<equiv> valid_eqns ES \<and> finite ES \<and> distinct_equas ES \<and> ardenable ES \<and> 
+                non_empty ES \<and> finite_rhs ES \<and> self_contained ES"
+
+subsection {* The proof of this direction *}
+
+subsubsection {* Basic properties *}
+
+text {*
+  The following are some basic properties of the above definitions.
+*}
+
+lemma L_rhs_union_distrib:
+  " L (A::rhs_item set) \<union> L B = L (A \<union> B)"
+by simp
+
+lemma finite_snd_Trn:
+  assumes finite:"finite rhs"
+  shows "finite {r\<^isub>2. Trn Y r\<^isub>2 \<in> rhs}" (is "finite ?B")
+proof-
+  def rhs' \<equiv> "{e \<in> rhs. \<exists> r. e = Trn Y r}"
+  have "?B = (snd o the_Trn) ` rhs'" using rhs'_def by (auto simp:image_def)
+  moreover have "finite rhs'" using finite rhs'_def by auto
+  ultimately show ?thesis by simp
+qed
+
+lemma rexp_of_empty:
+  assumes finite:"finite rhs"
+  and nonempty:"rhs_nonempty rhs"
+  shows "[] \<notin> L (rexp_of rhs X)"
+using finite nonempty rhs_nonempty_def
+by (drule_tac finite_snd_Trn[where Y = X], auto simp:rexp_of_def items_of_def)
+
+lemma [intro!]:
+  "P (Trn X r) \<Longrightarrow> (\<exists>a. (\<exists>r. a = Trn X r \<and> P a))" by auto
+
+lemma finite_items_of:
+  "finite rhs \<Longrightarrow> finite (items_of rhs X)"
+by (auto simp:items_of_def intro:finite_subset)
+
+lemma lang_of_rexp_of:
+  assumes finite:"finite rhs"
+  shows "L (items_of rhs X) = X ;; (L (rexp_of rhs X))"
+proof -
+  have "finite ((snd \<circ> the_Trn) ` items_of rhs X)" using finite_items_of[OF finite] by auto
+  thus ?thesis
+    apply (auto simp:rexp_of_def Seq_def items_of_def)
+    apply (rule_tac x = s1 in exI, rule_tac x = s2 in exI, auto)
+    by (rule_tac x= "Trn X r" in exI, auto simp:Seq_def)
+qed
+
+lemma rexp_of_lam_eq_lam_set:
+  assumes finite: "finite rhs"
+  shows "L (rexp_of_lam rhs) = L (lam_of rhs)"
+proof -
+  have "finite (the_r ` {Lam r |r. Lam r \<in> rhs})" using finite
+    by (rule_tac finite_imageI, auto intro:finite_subset)
+  thus ?thesis by (auto simp:rexp_of_lam_def lam_of_def)
+qed
+
+lemma [simp]:
+  " L (attach_rexp r xb) = L xb ;; L r"
+apply (cases xb, auto simp:Seq_def)
+by (rule_tac x = "s1 @ s1a" in exI, rule_tac x = s2a in exI,auto simp:Seq_def)
+
+lemma lang_of_append_rhs:
+  "L (append_rhs_rexp rhs r) = L rhs ;; L r"
+apply (auto simp:append_rhs_rexp_def image_def)
+apply (auto simp:Seq_def)
+apply (rule_tac x = "L xb ;; L r" in exI, auto simp add:Seq_def)
+by (rule_tac x = "attach_rexp r xb" in exI, auto simp:Seq_def)
+
+lemma classes_of_union_distrib:
+  "classes_of A \<union> classes_of B = classes_of (A \<union> B)"
+by (auto simp add:classes_of_def)
+
+lemma lefts_of_union_distrib:
+  "lefts_of A \<union> lefts_of B = lefts_of (A \<union> B)"
+by (auto simp:lefts_of_def)
+
+
+subsubsection {* Intialization *}
+
+text {*
+  The following several lemmas until @{text "init_ES_satisfy_Inv"} shows that
+  the initial equational system satisfies invariant @{text "Inv"}.
+  *}
+
+lemma defined_by_str:
+  "\<lbrakk>s \<in> X; X \<in> UNIV // (\<approx>Lang)\<rbrakk> \<Longrightarrow> X = (\<approx>Lang) `` {s}"
+by (auto simp:quotient_def Image_def str_eq_rel_def)
+
+lemma every_eqclass_has_transition:
+  assumes has_str: "s @ [c] \<in> X"
+  and     in_CS:   "X \<in> UNIV // (\<approx>Lang)"
+  obtains Y where "Y \<in> UNIV // (\<approx>Lang)" and "Y ;; {[c]} \<subseteq> X" and "s \<in> Y"
+proof -
+  def Y \<equiv> "(\<approx>Lang) `` {s}"
+  have "Y \<in> UNIV // (\<approx>Lang)" 
+    unfolding Y_def quotient_def by auto
+  moreover
+  have "X = (\<approx>Lang) `` {s @ [c]}" 
+    using has_str in_CS defined_by_str by blast
+  then have "Y ;; {[c]} \<subseteq> X" 
+    unfolding Y_def Image_def Seq_def
+    unfolding str_eq_rel_def
+    by clarsimp
+  moreover
+  have "s \<in> Y" unfolding Y_def 
+    unfolding Image_def str_eq_rel_def by simp
+  ultimately show thesis by (blast intro: that)
+qed
+
+lemma l_eq_r_in_eqs:
+  assumes X_in_eqs: "(X, xrhs) \<in> (eqs (UNIV // (\<approx>Lang)))"
+  shows "X = L xrhs"
+proof 
+  show "X \<subseteq> L xrhs"
+  proof
+    fix x
+    assume "(1)": "x \<in> X"
+    show "x \<in> L xrhs"          
+    proof (cases "x = []")
+      assume empty: "x = []"
+      thus ?thesis using X_in_eqs "(1)"
+        by (auto simp:eqs_def init_rhs_def)
+    next
+      assume not_empty: "x \<noteq> []"
+      then obtain clist c where decom: "x = clist @ [c]"
+        by (case_tac x rule:rev_cases, auto)
+      have "X \<in> UNIV // (\<approx>Lang)" using X_in_eqs by (auto simp:eqs_def)
+      then obtain Y 
+        where "Y \<in> UNIV // (\<approx>Lang)" 
+        and "Y ;; {[c]} \<subseteq> X"
+        and "clist \<in> Y"
+        using decom "(1)" every_eqclass_has_transition by blast
+      hence 
+        "x \<in> L {Trn Y (CHAR c)| Y c. Y \<in> UNIV // (\<approx>Lang) \<and> Y ;; {[c]} \<subseteq> X}"
+        using "(1)" decom
+        by (simp, rule_tac x = "Trn Y (CHAR c)" in exI, simp add:Seq_def)
+      thus ?thesis using X_in_eqs "(1)"
+        by (simp add:eqs_def init_rhs_def)
+    qed
+  qed
+next
+  show "L xrhs \<subseteq> X" using X_in_eqs
+    by (auto simp:eqs_def init_rhs_def) 
+qed
+
+lemma finite_init_rhs: 
+  assumes finite: "finite CS"
+  shows "finite (init_rhs CS X)"
+proof-
+  have "finite {Trn Y (CHAR c) |Y c. Y \<in> CS \<and> Y ;; {[c]} \<subseteq> X}" (is "finite ?A")
+  proof -
+    def S \<equiv> "{(Y, c)| Y c. Y \<in> CS \<and> Y ;; {[c]} \<subseteq> X}" 
+    def h \<equiv> "\<lambda> (Y, c). Trn Y (CHAR c)"
+    have "finite (CS \<times> (UNIV::char set))" using finite by auto
+    hence "finite S" using S_def 
+      by (rule_tac B = "CS \<times> UNIV" in finite_subset, auto)
+    moreover have "?A = h ` S" by (auto simp: S_def h_def image_def)
+    ultimately show ?thesis 
+      by auto
+  qed
+  thus ?thesis by (simp add:init_rhs_def)
+qed
+
+lemma init_ES_satisfy_Inv:
+  assumes finite_CS: "finite (UNIV // (\<approx>Lang))"
+  shows "Inv (eqs (UNIV // (\<approx>Lang)))"
+proof -
+  have "finite (eqs (UNIV // (\<approx>Lang)))" using finite_CS
+    by (simp add:eqs_def)
+  moreover have "distinct_equas (eqs (UNIV // (\<approx>Lang)))"     
+    by (simp add:distinct_equas_def eqs_def)
+  moreover have "ardenable (eqs (UNIV // (\<approx>Lang)))"
+    by (auto simp add:ardenable_def eqs_def init_rhs_def rhs_nonempty_def del:L_rhs.simps)
+  moreover have "valid_eqns (eqs (UNIV // (\<approx>Lang)))"
+    using l_eq_r_in_eqs by (simp add:valid_eqns_def)
+  moreover have "non_empty (eqs (UNIV // (\<approx>Lang)))"
+    by (auto simp:non_empty_def eqs_def quotient_def Image_def str_eq_rel_def)
+  moreover have "finite_rhs (eqs (UNIV // (\<approx>Lang)))"
+    using finite_init_rhs[OF finite_CS] 
+    by (auto simp:finite_rhs_def eqs_def)
+  moreover have "self_contained (eqs (UNIV // (\<approx>Lang)))"
+    by (auto simp:self_contained_def eqs_def init_rhs_def classes_of_def lefts_of_def)
+  ultimately show ?thesis by (simp add:Inv_def)
+qed
+
+subsubsection {* 
+  Interation step
+  *}
+
+text {*
+  From this point until @{text "iteration_step"}, it is proved
+  that there exists iteration steps which keep @{text "Inv(ES)"} while
+  decreasing the size of @{text "ES"}.
+  *}
+lemma arden_variate_keeps_eq:
+  assumes l_eq_r: "X = L rhs"
+  and not_empty: "[] \<notin> L (rexp_of rhs X)"
+  and finite: "finite rhs"
+  shows "X = L (arden_variate X rhs)"
+proof -
+  def A \<equiv> "L (rexp_of rhs X)"
+  def b \<equiv> "rhs - items_of rhs X"
+  def B \<equiv> "L b" 
+  have "X = B ;; A\<star>"
+  proof-
+    have "rhs = items_of rhs X \<union> b" by (auto simp:b_def items_of_def)
+    hence "L rhs = L(items_of rhs X \<union> b)" by simp
+    hence "L rhs = L(items_of rhs X) \<union> B" by (simp only:L_rhs_union_distrib B_def)
+    with lang_of_rexp_of
+    have "L rhs = X ;; A \<union> B " using finite by (simp only:B_def b_def A_def)
+    thus ?thesis
+      using l_eq_r not_empty
+      apply (drule_tac B = B and X = X in ardens_revised)
+      by (auto simp:A_def simp del:L_rhs.simps)
+  qed
+  moreover have "L (arden_variate X rhs) = (B ;; A\<star>)" (is "?L = ?R")
+    by (simp only:arden_variate_def L_rhs_union_distrib lang_of_append_rhs 
+                  B_def A_def b_def L_rexp.simps seq_union_distrib)
+   ultimately show ?thesis by simp
+qed 
+
+lemma append_keeps_finite:
+  "finite rhs \<Longrightarrow> finite (append_rhs_rexp rhs r)"
+by (auto simp:append_rhs_rexp_def)
+
+lemma arden_variate_keeps_finite:
+  "finite rhs \<Longrightarrow> finite (arden_variate X rhs)"
+by (auto simp:arden_variate_def append_keeps_finite)
+
+lemma append_keeps_nonempty:
+  "rhs_nonempty rhs \<Longrightarrow> rhs_nonempty (append_rhs_rexp rhs r)"
+apply (auto simp:rhs_nonempty_def append_rhs_rexp_def)
+by (case_tac x, auto simp:Seq_def)
+
+lemma nonempty_set_sub:
+  "rhs_nonempty rhs \<Longrightarrow> rhs_nonempty (rhs - A)"
+by (auto simp:rhs_nonempty_def)
+
+lemma nonempty_set_union:
+  "\<lbrakk>rhs_nonempty rhs; rhs_nonempty rhs'\<rbrakk> \<Longrightarrow> rhs_nonempty (rhs \<union> rhs')"
+by (auto simp:rhs_nonempty_def)
+
+lemma arden_variate_keeps_nonempty:
+  "rhs_nonempty rhs \<Longrightarrow> rhs_nonempty (arden_variate X rhs)"
+by (simp only:arden_variate_def append_keeps_nonempty nonempty_set_sub)
+
+
+lemma rhs_subst_keeps_nonempty:
+  "\<lbrakk>rhs_nonempty rhs; rhs_nonempty xrhs\<rbrakk> \<Longrightarrow> rhs_nonempty (rhs_subst rhs X xrhs)"
+by (simp only:rhs_subst_def append_keeps_nonempty  nonempty_set_union nonempty_set_sub)
+
+lemma rhs_subst_keeps_eq:
+  assumes substor: "X = L xrhs"
+  and finite: "finite rhs"
+  shows "L (rhs_subst rhs X xrhs) = L rhs" (is "?Left = ?Right")
+proof-
+  def A \<equiv> "L (rhs - items_of rhs X)"
+  have "?Left = A \<union> L (append_rhs_rexp xrhs (rexp_of rhs X))"
+    by (simp only:rhs_subst_def L_rhs_union_distrib A_def)
+  moreover have "?Right = A \<union> L (items_of rhs X)"
+  proof-
+    have "rhs = (rhs - items_of rhs X) \<union> (items_of rhs X)" by (auto simp:items_of_def)
+    thus ?thesis by (simp only:L_rhs_union_distrib A_def)
+  qed
+  moreover have "L (append_rhs_rexp xrhs (rexp_of rhs X)) = L (items_of rhs X)" 
+    using finite substor  by (simp only:lang_of_append_rhs lang_of_rexp_of)
+  ultimately show ?thesis by simp
+qed
+
+lemma rhs_subst_keeps_finite_rhs:
+  "\<lbrakk>finite rhs; finite yrhs\<rbrakk> \<Longrightarrow> finite (rhs_subst rhs Y yrhs)"
+by (auto simp:rhs_subst_def append_keeps_finite)
+
+lemma eqs_subst_keeps_finite:
+  assumes finite:"finite (ES:: (string set \<times> rhs_item set) set)"
+  shows "finite (eqs_subst ES Y yrhs)"
+proof -
+  have "finite {(Ya, rhs_subst yrhsa Y yrhs) |Ya yrhsa. (Ya, yrhsa) \<in> ES}" 
+                                                                  (is "finite ?A")
+  proof-
+    def eqns' \<equiv> "{((Ya::string set), yrhsa)| Ya yrhsa. (Ya, yrhsa) \<in> ES}"
+    def h \<equiv> "\<lambda> ((Ya::string set), yrhsa). (Ya, rhs_subst yrhsa Y yrhs)"
+    have "finite (h ` eqns')" using finite h_def eqns'_def by auto
+    moreover have "?A = h ` eqns'" by (auto simp:h_def eqns'_def)
+    ultimately show ?thesis by auto      
+  qed
+  thus ?thesis by (simp add:eqs_subst_def)
+qed
+
+lemma eqs_subst_keeps_finite_rhs:
+  "\<lbrakk>finite_rhs ES; finite yrhs\<rbrakk> \<Longrightarrow> finite_rhs (eqs_subst ES Y yrhs)"
+by (auto intro:rhs_subst_keeps_finite_rhs simp add:eqs_subst_def finite_rhs_def)
+
+lemma append_rhs_keeps_cls:
+  "classes_of (append_rhs_rexp rhs r) = classes_of rhs"
+apply (auto simp:classes_of_def append_rhs_rexp_def)
+apply (case_tac xa, auto simp:image_def)
+by (rule_tac x = "SEQ ra r" in exI, rule_tac x = "Trn x ra" in bexI, simp+)
+
+lemma arden_variate_removes_cl:
+  "classes_of (arden_variate Y yrhs) = classes_of yrhs - {Y}"
+apply (simp add:arden_variate_def append_rhs_keeps_cls items_of_def)
+by (auto simp:classes_of_def)
+
+lemma lefts_of_keeps_cls:
+  "lefts_of (eqs_subst ES Y yrhs) = lefts_of ES"
+by (auto simp:lefts_of_def eqs_subst_def)
+
+lemma rhs_subst_updates_cls:
+  "X \<notin> classes_of xrhs \<Longrightarrow> 
+      classes_of (rhs_subst rhs X xrhs) = classes_of rhs \<union> classes_of xrhs - {X}"
+apply (simp only:rhs_subst_def append_rhs_keeps_cls 
+                              classes_of_union_distrib[THEN sym])
+by (auto simp:classes_of_def items_of_def)
+
+lemma eqs_subst_keeps_self_contained:
+  fixes Y
+  assumes sc: "self_contained (ES \<union> {(Y, yrhs)})" (is "self_contained ?A")
+  shows "self_contained (eqs_subst ES Y (arden_variate Y yrhs))" 
+                                                   (is "self_contained ?B")
+proof-
+  { fix X xrhs'
+    assume "(X, xrhs') \<in> ?B"
+    then obtain xrhs 
+      where xrhs_xrhs': "xrhs' = rhs_subst xrhs Y (arden_variate Y yrhs)"
+      and X_in: "(X, xrhs) \<in> ES" by (simp add:eqs_subst_def, blast)    
+    have "classes_of xrhs' \<subseteq> lefts_of ?B"
+    proof-
+      have "lefts_of ?B = lefts_of ES" by (auto simp add:lefts_of_def eqs_subst_def)
+      moreover have "classes_of xrhs' \<subseteq> lefts_of ES"
+      proof-
+        have "classes_of xrhs' \<subseteq> 
+                        classes_of xrhs \<union> classes_of (arden_variate Y yrhs) - {Y}"
+        proof-
+          have "Y \<notin> classes_of (arden_variate Y yrhs)" 
+            using arden_variate_removes_cl by simp
+          thus ?thesis using xrhs_xrhs' by (auto simp:rhs_subst_updates_cls)
+        qed
+        moreover have "classes_of xrhs \<subseteq> lefts_of ES \<union> {Y}" using X_in sc
+          apply (simp only:self_contained_def lefts_of_union_distrib[THEN sym])
+          by (drule_tac x = "(X, xrhs)" in bspec, auto simp:lefts_of_def)
+        moreover have "classes_of (arden_variate Y yrhs) \<subseteq> lefts_of ES \<union> {Y}" 
+          using sc 
+          by (auto simp add:arden_variate_removes_cl self_contained_def lefts_of_def)
+        ultimately show ?thesis by auto
+      qed
+      ultimately show ?thesis by simp
+    qed
+  } thus ?thesis by (auto simp only:eqs_subst_def self_contained_def)
+qed
+
+lemma eqs_subst_satisfy_Inv:
+  assumes Inv_ES: "Inv (ES \<union> {(Y, yrhs)})"
+  shows "Inv (eqs_subst ES Y (arden_variate Y yrhs))"
+proof -  
+  have finite_yrhs: "finite yrhs" 
+    using Inv_ES by (auto simp:Inv_def finite_rhs_def)
+  have nonempty_yrhs: "rhs_nonempty yrhs" 
+    using Inv_ES by (auto simp:Inv_def ardenable_def)
+  have Y_eq_yrhs: "Y = L yrhs" 
+    using Inv_ES by (simp only:Inv_def valid_eqns_def, blast)
+  have "distinct_equas (eqs_subst ES Y (arden_variate Y yrhs))" 
+    using Inv_ES
+    by (auto simp:distinct_equas_def eqs_subst_def Inv_def)
+  moreover have "finite (eqs_subst ES Y (arden_variate Y yrhs))" 
+    using Inv_ES by (simp add:Inv_def eqs_subst_keeps_finite)
+  moreover have "finite_rhs (eqs_subst ES Y (arden_variate Y yrhs))"
+  proof-
+    have "finite_rhs ES" using Inv_ES 
+      by (simp add:Inv_def finite_rhs_def)
+    moreover have "finite (arden_variate Y yrhs)"
+    proof -
+      have "finite yrhs" using Inv_ES 
+        by (auto simp:Inv_def finite_rhs_def)
+      thus ?thesis using arden_variate_keeps_finite by simp
+    qed
+    ultimately show ?thesis 
+      by (simp add:eqs_subst_keeps_finite_rhs)
+  qed
+  moreover have "ardenable (eqs_subst ES Y (arden_variate Y yrhs))"
+  proof - 
+    { fix X rhs
+      assume "(X, rhs) \<in> ES"
+      hence "rhs_nonempty rhs"  using prems Inv_ES  
+        by (simp add:Inv_def ardenable_def)
+      with nonempty_yrhs 
+      have "rhs_nonempty (rhs_subst rhs Y (arden_variate Y yrhs))"
+        by (simp add:nonempty_yrhs 
+               rhs_subst_keeps_nonempty arden_variate_keeps_nonempty)
+    } thus ?thesis by (auto simp add:ardenable_def eqs_subst_def)
+  qed
+  moreover have "valid_eqns (eqs_subst ES Y (arden_variate Y yrhs))"
+  proof-
+    have "Y = L (arden_variate Y yrhs)" 
+      using Y_eq_yrhs Inv_ES finite_yrhs nonempty_yrhs      
+      by (rule_tac arden_variate_keeps_eq, (simp add:rexp_of_empty)+)
+    thus ?thesis using Inv_ES 
+      by (clarsimp simp add:valid_eqns_def 
+              eqs_subst_def rhs_subst_keeps_eq Inv_def finite_rhs_def
+                   simp del:L_rhs.simps)
+  qed
+  moreover have 
+    non_empty_subst: "non_empty (eqs_subst ES Y (arden_variate Y yrhs))"
+    using Inv_ES by (auto simp:Inv_def non_empty_def eqs_subst_def)
+  moreover 
+  have self_subst: "self_contained (eqs_subst ES Y (arden_variate Y yrhs))"
+    using Inv_ES eqs_subst_keeps_self_contained by (simp add:Inv_def)
+  ultimately show ?thesis using Inv_ES by (simp add:Inv_def)
+qed
+
+lemma eqs_subst_card_le: 
+  assumes finite: "finite (ES::(string set \<times> rhs_item set) set)"
+  shows "card (eqs_subst ES Y yrhs) <= card ES"
+proof-
+  def f \<equiv> "\<lambda> x. ((fst x)::string set, rhs_subst (snd x) Y yrhs)"
+  have "eqs_subst ES Y yrhs = f ` ES" 
+    apply (auto simp:eqs_subst_def f_def image_def)
+    by (rule_tac x = "(Ya, yrhsa)" in bexI, simp+)
+  thus ?thesis using finite by (auto intro:card_image_le)
+qed
+
+lemma eqs_subst_cls_remains: 
+  "(X, xrhs) \<in> ES \<Longrightarrow> \<exists> xrhs'. (X, xrhs') \<in> (eqs_subst ES Y yrhs)"
+by (auto simp:eqs_subst_def)
+
+lemma card_noteq_1_has_more:
+  assumes card:"card S \<noteq> 1"
+  and e_in: "e \<in> S"
+  and finite: "finite S"
+  obtains e' where "e' \<in> S \<and> e \<noteq> e'" 
+proof-
+  have "card (S - {e}) > 0"
+  proof -
+    have "card S > 1" using card e_in finite  
+      by (case_tac "card S", auto) 
+    thus ?thesis using finite e_in by auto
+  qed
+  hence "S - {e} \<noteq> {}" using finite by (rule_tac notI, simp)
+  thus "(\<And>e'. e' \<in> S \<and> e \<noteq> e' \<Longrightarrow> thesis) \<Longrightarrow> thesis" by auto
+qed
+
+lemma iteration_step: 
+  assumes Inv_ES: "Inv ES"
+  and    X_in_ES: "(X, xrhs) \<in> ES"
+  and    not_T: "card ES \<noteq> 1"
+  shows "\<exists> ES'. (Inv ES' \<and> (\<exists> xrhs'.(X, xrhs') \<in> ES')) \<and> 
+                (card ES', card ES) \<in> less_than" (is "\<exists> ES'. ?P ES'")
+proof -
+  have finite_ES: "finite ES" using Inv_ES by (simp add:Inv_def)
+  then obtain Y yrhs 
+    where Y_in_ES: "(Y, yrhs) \<in> ES" and not_eq: "(X, xrhs) \<noteq> (Y, yrhs)" 
+    using not_T X_in_ES by (drule_tac card_noteq_1_has_more, auto)
+  def ES' == "ES - {(Y, yrhs)}"
+  let ?ES'' = "eqs_subst ES' Y (arden_variate Y yrhs)"
+  have "?P ?ES''"
+  proof -
+    have "Inv ?ES''" using Y_in_ES Inv_ES
+      by (rule_tac eqs_subst_satisfy_Inv, simp add:ES'_def insert_absorb)
+    moreover have "\<exists>xrhs'. (X, xrhs') \<in> ?ES''"  using not_eq X_in_ES
+      by (rule_tac ES = ES' in eqs_subst_cls_remains, auto simp add:ES'_def)
+    moreover have "(card ?ES'', card ES) \<in> less_than" 
+    proof -
+      have "finite ES'" using finite_ES ES'_def by auto
+      moreover have "card ES' < card ES" using finite_ES Y_in_ES
+        by (auto simp:ES'_def card_gt_0_iff intro:diff_Suc_less)
+      ultimately show ?thesis 
+        by (auto dest:eqs_subst_card_le elim:le_less_trans)
+    qed
+    ultimately show ?thesis by simp
+  qed
+  thus ?thesis by blast
+qed
+
+subsubsection {*
+  Conclusion of the proof
+  *}
+
+text {*
+  From this point until @{text "hard_direction"}, the hard direction is proved
+  through a simple application of the iteration principle.
+*}
+
+lemma iteration_conc: 
+  assumes history: "Inv ES"
+  and    X_in_ES: "\<exists> xrhs. (X, xrhs) \<in> ES"
+  shows 
+  "\<exists> ES'. (Inv ES' \<and> (\<exists> xrhs'. (X, xrhs') \<in> ES')) \<and> card ES' = 1" 
+                                                          (is "\<exists> ES'. ?P ES'")
+proof (cases "card ES = 1")
+  case True
+  thus ?thesis using history X_in_ES
+    by blast
+next
+  case False  
+  thus ?thesis using history iteration_step X_in_ES
+    by (rule_tac f = card in wf_iter, auto)
+qed
+  
+lemma last_cl_exists_rexp:
+  assumes ES_single: "ES = {(X, xrhs)}" 
+  and Inv_ES: "Inv ES"
+  shows "\<exists> (r::rexp). L r = X" (is "\<exists> r. ?P r")
+proof-
+  let ?A = "arden_variate X xrhs"
+  have "?P (rexp_of_lam ?A)"
+  proof -
+    have "L (rexp_of_lam ?A) = L (lam_of ?A)"
+    proof(rule rexp_of_lam_eq_lam_set)
+      show "finite (arden_variate X xrhs)" using Inv_ES ES_single 
+        by (rule_tac arden_variate_keeps_finite, 
+                       auto simp add:Inv_def finite_rhs_def)
+    qed
+    also have "\<dots> = L ?A"
+    proof-
+      have "lam_of ?A = ?A"
+      proof-
+        have "classes_of ?A = {}" using Inv_ES ES_single
+          by (simp add:arden_variate_removes_cl 
+                       self_contained_def Inv_def lefts_of_def) 
+        thus ?thesis 
+          by (auto simp only:lam_of_def classes_of_def, case_tac x, auto)
+      qed
+      thus ?thesis by simp
+    qed
+    also have "\<dots> = X"
+    proof(rule arden_variate_keeps_eq [THEN sym])
+      show "X = L xrhs" using Inv_ES ES_single 
+        by (auto simp only:Inv_def valid_eqns_def)  
+    next
+      from Inv_ES ES_single show "[] \<notin> L (rexp_of xrhs X)"
+        by(simp add:Inv_def ardenable_def rexp_of_empty finite_rhs_def)
+    next
+      from Inv_ES ES_single show "finite xrhs" 
+        by (simp add:Inv_def finite_rhs_def)
+    qed
+    finally show ?thesis by simp
+  qed
+  thus ?thesis by auto
+qed
+   
+lemma every_eqcl_has_reg: 
+  assumes finite_CS: "finite (UNIV // (\<approx>Lang))"
+  and X_in_CS: "X \<in> (UNIV // (\<approx>Lang))"
+  shows "\<exists> (reg::rexp). L reg = X" (is "\<exists> r. ?E r")
+proof -
+  from X_in_CS have "\<exists> xrhs. (X, xrhs) \<in> (eqs (UNIV  // (\<approx>Lang)))"
+    by (auto simp:eqs_def init_rhs_def)
+  then obtain ES xrhs where Inv_ES: "Inv ES" 
+    and X_in_ES: "(X, xrhs) \<in> ES"
+    and card_ES: "card ES = 1"
+    using finite_CS X_in_CS init_ES_satisfy_Inv iteration_conc
+    by blast
+  hence ES_single_equa: "ES = {(X, xrhs)}" 
+    by (auto simp:Inv_def dest!:card_Suc_Diff1 simp:card_eq_0_iff) 
+  thus ?thesis using Inv_ES
+    by (rule last_cl_exists_rexp)
+qed
+
+lemma finals_in_partitions:
+  "finals Lang \<subseteq> (UNIV // (\<approx>Lang))"
+  by (auto simp:finals_def quotient_def)   
+
+theorem hard_direction: 
+  assumes finite_CS: "finite (UNIV // (\<approx>Lang))"
+  shows   "\<exists> (reg::rexp). Lang = L reg"
+proof -
+  have "\<forall> X \<in> (UNIV // (\<approx>Lang)). \<exists> (reg::rexp). X = L reg" 
+    using finite_CS every_eqcl_has_reg by blast
+  then obtain f 
+    where f_prop: "\<forall> X \<in> (UNIV // (\<approx>Lang)). X = L ((f X)::rexp)" 
+    by (auto dest:bchoice)
+  def rs \<equiv> "f ` (finals Lang)"  
+  have "Lang = \<Union> (finals Lang)" using lang_is_union_of_finals by auto
+  also have "\<dots> = L (folds ALT NULL rs)" 
+  proof -
+    have "finite rs"
+    proof -
+      have "finite (finals Lang)" 
+        using finite_CS finals_in_partitions[of "Lang"]   
+        by (erule_tac finite_subset, simp)
+      thus ?thesis using rs_def by auto
+    qed
+    thus ?thesis 
+      using f_prop rs_def finals_in_partitions[of "Lang"] by auto
+  qed
+  finally show ?thesis by blast
+qed 
+
+section {* Direction: @{text "regular language \<Rightarrow>finite partition"} *}
+
+subsection {* The scheme for this direction *}
+
+text {* 
+  The following convenient notation @{text "x \<approx>Lang y"} means:
+  string @{text "x"} and @{text "y"} are equivalent with respect to 
+  language @{text "Lang"}.
+  *}
+
+definition
+  str_eq ("_ \<approx>_ _")
+where
+  "x \<approx>Lang y \<equiv> (x, y) \<in> (\<approx>Lang)"
+
+text {*
+  The very basic scheme to show the finiteness of the partion generated by a language @{text "Lang"}
+  is by attaching tags to every string. The set of tags are carfully choosen to make it finite.
+  If it can be proved that strings with the same tag are equivlent with respect @{text "Lang"},
+  then the partition given rise by @{text "Lang"} must be finite. The reason for this is a lemma 
+  in standard library (@{text "finite_imageD"}), which says: if the image of an injective 
+  function on a set @{text "A"} is finite, then @{text "A"} is finite. It can be shown that
+  the function obtained by llifting @{text "tag"}
+  to the level of equalent classes (i.e. @{text "((op `) tag)"}) is injective 
+  (by lemma @{text "tag_image_injI"}) and the image of this function is finite 
+  (with the help of lemma @{text "finite_tag_imageI"}). This argument is formalized
+  by the following lemma @{text "tag_finite_imageD"}.
+  *}
+
+lemma tag_finite_imageD:
+  assumes str_inj: "\<And> m n. tag m = tag (n::string) \<Longrightarrow> m \<approx>lang n"
+  and range: "finite (range tag)"
+  shows "finite (UNIV // (\<approx>lang))"
+proof (rule_tac f = "(op `) tag" in finite_imageD)
+  show "finite (op ` tag ` UNIV // \<approx>lang)" using range
+    apply (rule_tac B = "Pow (tag ` UNIV)" in finite_subset)
+    by (auto simp add:image_def Pow_def)
+next
+  show "inj_on (op ` tag) (UNIV // \<approx>lang)" 
+  proof-
+    { fix X Y
+      assume X_in: "X \<in> UNIV // \<approx>lang"
+        and  Y_in: "Y \<in> UNIV // \<approx>lang"
+        and  tag_eq: "tag ` X = tag ` Y"
+      then obtain x y where "x \<in> X" and "y \<in> Y" and "tag x = tag y"
+        unfolding quotient_def Image_def str_eq_rel_def str_eq_def image_def
+        apply simp by blast
+      with X_in Y_in str_inj[of x y]
+      have "X = Y" by (auto simp:quotient_def str_eq_rel_def str_eq_def) 
+    } thus ?thesis unfolding inj_on_def by auto
+  qed
+qed
+
+subsection {* Lemmas for basic cases *}
+
+text {*
+  The the final result of this direction is in @{text "easier_direction"}, which
+  is an induction on the structure of regular expressions. There is one case 
+  for each regular expression operator. For basic operators such as @{text "NULL, EMPTY, CHAR c"},
+  the finiteness of their language partition can be established directly with no need
+  of taggiing. This section contains several technical lemma for these base cases.
+
+  The inductive cases involve operators @{text "ALT, SEQ"} and @{text "STAR"}. 
+  Tagging functions need to be defined individually for each of them. There will be one
+  dedicated section for each of these cases, and each section goes virtually the same way:
+  gives definition of the tagging function and prove that strings 
+  with the same tag are equivalent.
+  *}
+
+lemma quot_empty_subset:
+  "UNIV // (\<approx>{[]}) \<subseteq> {{[]}, UNIV - {[]}}"
+proof
+  fix x
+  assume "x \<in> UNIV // \<approx>{[]}"
+  then obtain y where h: "x = {z. (y, z) \<in> \<approx>{[]}}" 
+    unfolding quotient_def Image_def by blast
+  show "x \<in> {{[]}, UNIV - {[]}}" 
+  proof (cases "y = []")
+    case True with h
+    have "x = {[]}" by (auto simp:str_eq_rel_def)
+    thus ?thesis by simp
+  next
+    case False with h
+    have "x = UNIV - {[]}" by (auto simp:str_eq_rel_def)
+    thus ?thesis by simp
+  qed
+qed
+
+lemma quot_char_subset:
+  "UNIV // (\<approx>{[c]}) \<subseteq> {{[]},{[c]}, UNIV - {[], [c]}}"
+proof 
+  fix x 
+  assume "x \<in> UNIV // \<approx>{[c]}"
+  then obtain y where h: "x = {z. (y, z) \<in> \<approx>{[c]}}" 
+    unfolding quotient_def Image_def by blast
+  show "x \<in> {{[]},{[c]}, UNIV - {[], [c]}}"
+  proof -
+    { assume "y = []" hence "x = {[]}" using h 
+        by (auto simp:str_eq_rel_def)
+    } moreover {
+      assume "y = [c]" hence "x = {[c]}" using h 
+        by (auto dest!:spec[where x = "[]"] simp:str_eq_rel_def)
+    } moreover {
+      assume "y \<noteq> []" and "y \<noteq> [c]"
+      hence "\<forall> z. (y @ z) \<noteq> [c]" by (case_tac y, auto)
+      moreover have "\<And> p. (p \<noteq> [] \<and> p \<noteq> [c]) = (\<forall> q. p @ q \<noteq> [c])" 
+        by (case_tac p, auto)
+      ultimately have "x = UNIV - {[],[c]}" using h
+        by (auto simp add:str_eq_rel_def)
+    } ultimately show ?thesis by blast
+  qed
+qed
+
+subsection {* The case for @{text "SEQ"}*}
+
+definition 
+  "tag_str_SEQ L\<^isub>1 L\<^isub>2 x \<equiv> 
+       ((\<approx>L\<^isub>1) `` {x}, {(\<approx>L\<^isub>2) `` {x - xa}| xa.  xa \<le> x \<and> xa \<in> L\<^isub>1})"
+
+lemma tag_str_seq_range_finite:
+  "\<lbrakk>finite (UNIV // \<approx>L\<^isub>1); finite (UNIV // \<approx>L\<^isub>2)\<rbrakk> 
+                              \<Longrightarrow> finite (range (tag_str_SEQ L\<^isub>1 L\<^isub>2))"
+apply (rule_tac B = "(UNIV // \<approx>L\<^isub>1) \<times> (Pow (UNIV // \<approx>L\<^isub>2))" in finite_subset)
+by (auto simp:tag_str_SEQ_def Image_def quotient_def split:if_splits)
+
+lemma append_seq_elim:
+  assumes "x @ y \<in> L\<^isub>1 ;; L\<^isub>2"
+  shows "(\<exists> xa \<le> x. xa \<in> L\<^isub>1 \<and> (x - xa) @ y \<in> L\<^isub>2) \<or> 
+          (\<exists> ya \<le> y. (x @ ya) \<in> L\<^isub>1 \<and> (y - ya) \<in> L\<^isub>2)"
+proof-
+  from assms obtain s\<^isub>1 s\<^isub>2 
+    where "x @ y = s\<^isub>1 @ s\<^isub>2" 
+    and in_seq: "s\<^isub>1 \<in> L\<^isub>1 \<and> s\<^isub>2 \<in> L\<^isub>2" 
+    by (auto simp:Seq_def)
+  hence "(x \<le> s\<^isub>1 \<and> (s\<^isub>1 - x) @ s\<^isub>2 = y) \<or> (s\<^isub>1 \<le> x \<and> (x - s\<^isub>1) @ y = s\<^isub>2)"
+    using app_eq_dest by auto
+  moreover have "\<lbrakk>x \<le> s\<^isub>1; (s\<^isub>1 - x) @ s\<^isub>2 = y\<rbrakk> \<Longrightarrow> 
+                       \<exists> ya \<le> y. (x @ ya) \<in> L\<^isub>1 \<and> (y - ya) \<in> L\<^isub>2" 
+    using in_seq by (rule_tac x = "s\<^isub>1 - x" in exI, auto elim:prefixE)
+  moreover have "\<lbrakk>s\<^isub>1 \<le> x; (x - s\<^isub>1) @ y = s\<^isub>2\<rbrakk> \<Longrightarrow> 
+                    \<exists> xa \<le> x. xa \<in> L\<^isub>1 \<and> (x - xa) @ y \<in> L\<^isub>2" 
+    using in_seq by (rule_tac x = s\<^isub>1 in exI, auto)
+  ultimately show ?thesis by blast
+qed
+
+lemma tag_str_SEQ_injI:
+  "tag_str_SEQ L\<^isub>1 L\<^isub>2 m = tag_str_SEQ L\<^isub>1 L\<^isub>2 n \<Longrightarrow> m \<approx>(L\<^isub>1 ;; L\<^isub>2) n"
+proof-
+  { fix x y z
+    assume xz_in_seq: "x @ z \<in> L\<^isub>1 ;; L\<^isub>2"
+    and tag_xy: "tag_str_SEQ L\<^isub>1 L\<^isub>2 x = tag_str_SEQ L\<^isub>1 L\<^isub>2 y"
+    have"y @ z \<in> L\<^isub>1 ;; L\<^isub>2" 
+    proof-
+      have "(\<exists> xa \<le> x. xa \<in> L\<^isub>1 \<and> (x - xa) @ z \<in> L\<^isub>2) \<or> 
+               (\<exists> za \<le> z. (x @ za) \<in> L\<^isub>1 \<and> (z - za) \<in> L\<^isub>2)"
+        using xz_in_seq append_seq_elim by simp
+      moreover {
+        fix xa
+        assume h1: "xa \<le> x" and h2: "xa \<in> L\<^isub>1" and h3: "(x - xa) @ z \<in> L\<^isub>2"
+        obtain ya where "ya \<le> y" and "ya \<in> L\<^isub>1" and "(y - ya) @ z \<in> L\<^isub>2" 
+        proof -
+          have "\<exists> ya.  ya \<le> y \<and> ya \<in> L\<^isub>1 \<and> (x - xa) \<approx>L\<^isub>2 (y - ya)"
+          proof -
+            have "{\<approx>L\<^isub>2 `` {x - xa} |xa. xa \<le> x \<and> xa \<in> L\<^isub>1} = 
+                  {\<approx>L\<^isub>2 `` {y - xa} |xa. xa \<le> y \<and> xa \<in> L\<^isub>1}" 
+                          (is "?Left = ?Right") 
+              using h1 tag_xy by (auto simp:tag_str_SEQ_def)
+            moreover have "\<approx>L\<^isub>2 `` {x - xa} \<in> ?Left" using h1 h2 by auto
+            ultimately have "\<approx>L\<^isub>2 `` {x - xa} \<in> ?Right" by simp
+            thus ?thesis by (auto simp:Image_def str_eq_rel_def str_eq_def)
+          qed
+          with prems show ?thesis by (auto simp:str_eq_rel_def str_eq_def)
+        qed
+        hence "y @ z \<in> L\<^isub>1 ;; L\<^isub>2" by (erule_tac prefixE, auto simp:Seq_def)          
+      } moreover {
+        fix za
+        assume h1: "za \<le> z" and h2: "(x @ za) \<in> L\<^isub>1" and h3: "z - za \<in> L\<^isub>2"
+        hence "y @ za \<in> L\<^isub>1"
+        proof-
+          have "\<approx>L\<^isub>1 `` {x} = \<approx>L\<^isub>1 `` {y}" 
+            using h1 tag_xy by (auto simp:tag_str_SEQ_def)
+          with h2 show ?thesis 
+            by (auto simp:Image_def str_eq_rel_def str_eq_def) 
+        qed
+        with h1 h3 have "y @ z \<in> L\<^isub>1 ;; L\<^isub>2" 
+          by (drule_tac A = L\<^isub>1 in seq_intro, auto elim:prefixE)
+      }
+      ultimately show ?thesis by blast
+    qed
+  } thus "tag_str_SEQ L\<^isub>1 L\<^isub>2 m = tag_str_SEQ L\<^isub>1 L\<^isub>2 n \<Longrightarrow> m \<approx>(L\<^isub>1 ;; L\<^isub>2) n" 
+    by (auto simp add: str_eq_def str_eq_rel_def)
+qed 
+
+lemma quot_seq_finiteI:
+  "\<lbrakk>finite (UNIV // \<approx>L\<^isub>1); finite (UNIV // \<approx>L\<^isub>2)\<rbrakk> 
+  \<Longrightarrow> finite (UNIV // \<approx>(L\<^isub>1 ;; L\<^isub>2))"
+  apply (rule_tac tag = "tag_str_SEQ L\<^isub>1 L\<^isub>2" in tag_finite_imageD)  
+  by (auto intro:tag_str_SEQ_injI elim:tag_str_seq_range_finite)
+
+subsection {* The case for @{text "ALT"} *}
+
+definition 
+  "tag_str_ALT L\<^isub>1 L\<^isub>2 (x::string) \<equiv> ((\<approx>L\<^isub>1) `` {x}, (\<approx>L\<^isub>2) `` {x})"
+
+lemma quot_union_finiteI:
+  assumes finite1: "finite (UNIV // \<approx>(L\<^isub>1::string set))"
+  and finite2: "finite (UNIV // \<approx>L\<^isub>2)"
+  shows "finite (UNIV // \<approx>(L\<^isub>1 \<union> L\<^isub>2))"
+proof (rule_tac tag = "tag_str_ALT L\<^isub>1 L\<^isub>2" in tag_finite_imageD)
+  show "\<And>m n. tag_str_ALT L\<^isub>1 L\<^isub>2 m = tag_str_ALT L\<^isub>1 L\<^isub>2 n \<Longrightarrow> m \<approx>(L\<^isub>1 \<union> L\<^isub>2) n"
+    unfolding tag_str_ALT_def str_eq_def Image_def str_eq_rel_def by auto
+next
+  show "finite (range (tag_str_ALT L\<^isub>1 L\<^isub>2))" using finite1 finite2
+    apply (rule_tac B = "(UNIV // \<approx>L\<^isub>1) \<times> (UNIV // \<approx>L\<^isub>2)" in finite_subset)
+    by (auto simp:tag_str_ALT_def Image_def quotient_def)
+qed
+
+subsection {*
+  The case for @{text "STAR"}
+  *}
+
+text {* 
+  This turned out to be the trickiest case. 
+  *} (* I will make some illustrations for it. *)
+
+definition 
+  "tag_str_STAR L\<^isub>1 x \<equiv> {(\<approx>L\<^isub>1) `` {x - xa} | xa. xa < x \<and> xa \<in> L\<^isub>1\<star>}"
+
+lemma finite_set_has_max: "\<lbrakk>finite A; A \<noteq> {}\<rbrakk> \<Longrightarrow> 
+           (\<exists> max \<in> A. \<forall> a \<in> A. f a <= (f max :: nat))"
+proof (induct rule:finite.induct)
+  case emptyI thus ?case by simp
+next
+  case (insertI A a)
+  show ?case
+  proof (cases "A = {}")
+    case True thus ?thesis by (rule_tac x = a in bexI, auto)
+  next
+    case False
+    with prems obtain max 
+      where h1: "max \<in> A" 
+      and h2: "\<forall>a\<in>A. f a \<le> f max" by blast
+    show ?thesis
+    proof (cases "f a \<le> f max")
+      assume "f a \<le> f max"
+      with h1 h2 show ?thesis by (rule_tac x = max in bexI, auto)
+    next
+      assume "\<not> (f a \<le> f max)"
+      thus ?thesis using h2 by (rule_tac x = a in bexI, auto)
+    qed
+  qed
+qed
+
+lemma finite_strict_prefix_set: "finite {xa. xa < (x::string)}"
+apply (induct x rule:rev_induct, simp)
+apply (subgoal_tac "{xa. xa < xs @ [x]} = {xa. xa < xs} \<union> {xs}")
+by (auto simp:strict_prefix_def)
+
+
+lemma tag_str_star_range_finite:
+  "finite (UNIV // \<approx>L\<^isub>1) \<Longrightarrow> finite (range (tag_str_STAR L\<^isub>1))"
+apply (rule_tac B = "Pow (UNIV // \<approx>L\<^isub>1)" in finite_subset)
+by (auto simp:tag_str_STAR_def Image_def 
+                       quotient_def split:if_splits)
+
+lemma tag_str_STAR_injI:
+  "tag_str_STAR L\<^isub>1 m = tag_str_STAR L\<^isub>1 n \<Longrightarrow> m \<approx>(L\<^isub>1\<star>) n"
+proof-
+  { fix x y z
+    assume xz_in_star: "x @ z \<in> L\<^isub>1\<star>"
+    and tag_xy: "tag_str_STAR L\<^isub>1 x = tag_str_STAR L\<^isub>1 y"
+    have "y @ z \<in> L\<^isub>1\<star>"
+    proof(cases "x = []")
+      case True
+      with tag_xy have "y = []" 
+        by (auto simp:tag_str_STAR_def strict_prefix_def)
+      thus ?thesis using xz_in_star True by simp
+    next
+      case False
+      obtain x_max 
+        where h1: "x_max < x" 
+        and h2: "x_max \<in> L\<^isub>1\<star>" 
+        and h3: "(x - x_max) @ z \<in> L\<^isub>1\<star>" 
+        and h4:"\<forall> xa < x. xa \<in> L\<^isub>1\<star> \<and> (x - xa) @ z \<in> L\<^isub>1\<star> 
+                                     \<longrightarrow> length xa \<le> length x_max"
+      proof-
+        let ?S = "{xa. xa < x \<and> xa \<in> L\<^isub>1\<star> \<and> (x - xa) @ z \<in> L\<^isub>1\<star>}"
+        have "finite ?S"
+          by (rule_tac B = "{xa. xa < x}" in finite_subset, 
+                                auto simp:finite_strict_prefix_set)
+        moreover have "?S \<noteq> {}" using False xz_in_star
+          by (simp, rule_tac x = "[]" in exI, auto simp:strict_prefix_def)
+        ultimately have "\<exists> max \<in> ?S. \<forall> a \<in> ?S. length a \<le> length max" 
+          using finite_set_has_max by blast
+        with prems show ?thesis by blast
+      qed
+      obtain ya 
+        where h5: "ya < y" and h6: "ya \<in> L\<^isub>1\<star>" and h7: "(x - x_max) \<approx>L\<^isub>1 (y - ya)"
+      proof-
+        from tag_xy have "{\<approx>L\<^isub>1 `` {x - xa} |xa. xa < x \<and> xa \<in> L\<^isub>1\<star>} = 
+          {\<approx>L\<^isub>1 `` {y - xa} |xa. xa < y \<and> xa \<in> L\<^isub>1\<star>}" (is "?left = ?right")
+          by (auto simp:tag_str_STAR_def)
+        moreover have "\<approx>L\<^isub>1 `` {x - x_max} \<in> ?left" using h1 h2 by auto
+        ultimately have "\<approx>L\<^isub>1 `` {x - x_max} \<in> ?right" by simp
+        with prems show ?thesis apply 
+          (simp add:Image_def str_eq_rel_def str_eq_def) by blast
+      qed      
+      have "(y - ya) @ z \<in> L\<^isub>1\<star>" 
+      proof-
+        from h3 h1 obtain a b where a_in: "a \<in> L\<^isub>1" 
+          and a_neq: "a \<noteq> []" and b_in: "b \<in> L\<^isub>1\<star>" 
+          and ab_max: "(x - x_max) @ z = a @ b" 
+          by (drule_tac star_decom, auto simp:strict_prefix_def elim:prefixE)
+        have "(x - x_max) \<le> a \<and> (a - (x - x_max)) @ b = z" 
+        proof -
+          have "((x - x_max) \<le> a \<and> (a - (x - x_max)) @ b = z) \<or> 
+                            (a < (x - x_max) \<and> ((x - x_max) - a) @ z = b)" 
+            using app_eq_dest[OF ab_max] by (auto simp:strict_prefix_def)
+          moreover { 
+            assume np: "a < (x - x_max)" and b_eqs: " ((x - x_max) - a) @ z = b"
+            have "False"
+            proof -
+              let ?x_max' = "x_max @ a"
+              have "?x_max' < x" 
+                using np h1 by (clarsimp simp:strict_prefix_def diff_prefix) 
+              moreover have "?x_max' \<in> L\<^isub>1\<star>" 
+                using a_in h2 by (simp add:star_intro3) 
+              moreover have "(x - ?x_max') @ z \<in> L\<^isub>1\<star>" 
+                using b_eqs b_in np h1 by (simp add:diff_diff_appd)
+              moreover have "\<not> (length ?x_max' \<le> length x_max)" 
+                using a_neq by simp
+              ultimately show ?thesis using h4 by blast
+            qed 
+          } ultimately show ?thesis by blast
+        qed
+        then obtain za where z_decom: "z = za @ b" 
+          and x_za: "(x - x_max) @ za \<in> L\<^isub>1" 
+          using a_in by (auto elim:prefixE)        
+        from x_za h7 have "(y - ya) @ za \<in> L\<^isub>1" 
+          by (auto simp:str_eq_def str_eq_rel_def)
+        with z_decom b_in show ?thesis by (auto dest!:step[of "(y - ya) @ za"])
+      qed
+      with h5 h6 show ?thesis 
+        by (drule_tac star_intro1, auto simp:strict_prefix_def elim:prefixE)
+    qed      
+  } thus "tag_str_STAR L\<^isub>1 m = tag_str_STAR L\<^isub>1 n \<Longrightarrow> m \<approx>(L\<^isub>1\<star>) n"
+    by (auto simp add:str_eq_def str_eq_rel_def)
+qed
+
+lemma quot_star_finiteI:
+  "finite (UNIV // \<approx>L\<^isub>1) \<Longrightarrow> finite (UNIV // \<approx>(L\<^isub>1\<star>))"
+  apply (rule_tac tag = "tag_str_STAR L\<^isub>1" in tag_finite_imageD)
+  by (auto intro:tag_str_STAR_injI elim:tag_str_star_range_finite)
+
+subsection {*
+  The main lemma
+  *}
+
+lemma easier_directio\<nu>:
+  "Lang = L (r::rexp) \<Longrightarrow> finite (UNIV // (\<approx>Lang))"
+proof (induct arbitrary:Lang rule:rexp.induct)
+  case NULL
+  have "UNIV // (\<approx>{}) \<subseteq> {UNIV} "
+    by (auto simp:quotient_def str_eq_rel_def str_eq_def)
+  with prems show "?case" by (auto intro:finite_subset)
+next
+  case EMPTY
+  have "UNIV // (\<approx>{[]}) \<subseteq> {{[]}, UNIV - {[]}}" 
+    by (rule quot_empty_subset)
+  with prems show ?case by (auto intro:finite_subset)
+next
+  case (CHAR c)
+  have "UNIV // (\<approx>{[c]}) \<subseteq> {{[]},{[c]}, UNIV - {[], [c]}}" 
+    by (rule quot_char_subset)
+  with prems show ?case by (auto intro:finite_subset)
+next
+  case (SEQ r\<^isub>1 r\<^isub>2)
+  have "\<lbrakk>finite (UNIV // \<approx>(L r\<^isub>1)); finite (UNIV // \<approx>(L r\<^isub>2))\<rbrakk> 
+            \<Longrightarrow> finite (UNIV // \<approx>(L r\<^isub>1 ;; L r\<^isub>2))"
+    by (erule quot_seq_finiteI, simp)
+  with prems show ?case by simp
+next
+  case (ALT r\<^isub>1 r\<^isub>2)
+  have "\<lbrakk>finite (UNIV // \<approx>(L r\<^isub>1)); finite (UNIV // \<approx>(L r\<^isub>2))\<rbrakk> 
+            \<Longrightarrow> finite (UNIV // \<approx>(L r\<^isub>1 \<union> L r\<^isub>2))"
+    by (erule quot_union_finiteI, simp)
+  with prems show ?case by simp  
+next
+  case (STAR r)
+  have "finite (UNIV // \<approx>(L r)) 
+            \<Longrightarrow> finite (UNIV // \<approx>((L r)\<star>))"
+    by (erule quot_star_finiteI)
+  with prems show ?case by simp
+qed 
+
+end
+