
diff -r 1ce04eb1c8ad -r 48b231495281 utm/turing_basic.thy
--- a/utm/turing_basic.thy	Sat Sep 29 12:38:12 2012 +0000
+++ b/utm/turing_basic.thy	Mon Oct 15 13:23:52 2012 +0000
@@ -1,747 +1,736 @@
-theory turing_basic
-imports Main
-begin
-
-section {* Basic definitions of Turing machine *}
-
-(* Title: Turing machine's definition and its charater
-   Author: Xu Jian <xujian817@hotmail.com>
-   Maintainer: Xu Jian
-*)
-
-text {*
-\label{description of turing machine}
-*}
-
-section {* Basic definitions of Turing machine *}
-
-(* Title: Turing machine's definition and its charater
-   Author: Xu Jian <xujian817@hotmail.com>
-   Maintainer: Xu Jian
-*)
-
-text {*
-  Actions of Turing machine (Abbreviated TM in the following* ).
-*}
-
-datatype taction = 
-  -- {* Write zero *}
-  W0 | 
-  -- {* Write one *}
-  W1 | 
-  -- {* Move left *}
-  L | 
-  -- {* Move right *}
-  R | 
-  -- {* Do nothing *}
-  Nop
-
-text {*
-  Tape contents in every block.
-*}
-
-datatype block = 
-  -- {* Blank *}
-  Bk | 
-  -- {* Occupied *}
-  Oc
-
-text {*
-  Tape is represented as a pair of lists $(L_{left}, L_{right})$,
-  where $L_left$, named {\em left list}, is used to represent
-  the tape to the left of RW-head and
-  $L_{right}$, named {\em right list}, is used to represent the tape
-  under and to the right of RW-head.
-*}
-
-type_synonym tape = "block list \<times> block list"
-
-text {* The state of turing machine.*}
-type_synonym tstate = nat
-
-text {*
-  Turing machine instruction is represented as a 
-  pair @{text "(action, next_state)"},
-  where @{text "action"} is the action to take at the current state 
-  and @{text "next_state"} is the next state the machine is getting into
-  after the action.
-*}
-type_synonym tinst = "taction \<times> tstate"
-
-text {*
-  Program of Turing machine is represented as a list of Turing instructions
-  and the execution of the program starts from the head of the list.
-  *}
-type_synonym tprog = "tinst list"
-
-
-text {*
-  Turing machine configuration, which consists of the current state 
-  and the tape.
-*}
-type_synonym t_conf = "tstate \<times> tape"
-
-fun nth_of ::  "'a list \<Rightarrow> nat \<Rightarrow> 'a option"
-  where
-  "nth_of xs n = (if n < length xs then Some (xs!n)
-                  else None)"
-
-text {*
-  The function used to fetech instruction out of Turing program.
-  *}
-
-fun fetch :: "tprog \<Rightarrow> tstate \<Rightarrow> block \<Rightarrow> tinst"
-  where
-  "fetch p s b = (if s = 0 then (Nop, 0) else
-                  case b of 
-                     Bk \<Rightarrow> case nth_of p (2 * (s - 1)) of
-                          Some i \<Rightarrow> i
-                        | None \<Rightarrow> (Nop, 0) 
-                   | Oc \<Rightarrow> case nth_of p (2 * (s - 1) +1) of
-                          Some i \<Rightarrow> i
-                        | None \<Rightarrow> (Nop, 0))"
-
-
-fun new_tape :: "taction \<Rightarrow> tape \<Rightarrow> tape"
-where 
-   "new_tape action (leftn, rightn) = (case action of
-                                         W0 \<Rightarrow> (leftn, Bk#(tl rightn)) |
-                                         W1 \<Rightarrow> (leftn, Oc#(tl rightn)) |
-                                         L  \<Rightarrow>  (if leftn = [] then (tl leftn, Bk#rightn)
-                                               else (tl leftn, (hd leftn) # rightn)) |
-                                         R  \<Rightarrow> if rightn = [] then (Bk#leftn,tl rightn) 
-                                               else ((hd rightn)#leftn, tl rightn) |
-                                         Nop \<Rightarrow> (leftn, rightn)
-                                       )"
-
-text {*
-  The one step function used to transfer Turing machine configuration.
-*}
-fun tstep :: "t_conf \<Rightarrow> tprog \<Rightarrow> t_conf"
-  where
-  "tstep c p = (let (s, l, r) = c in 
-                     let (ac, ns) = (fetch p s (case r of [] \<Rightarrow> Bk |     
-                                                               x # xs \<Rightarrow> x)) in
-                       (ns, new_tape ac (l, r)))"
-
-text {*
-  The many-step function.
-*}
-fun steps :: "t_conf \<Rightarrow> tprog \<Rightarrow> nat \<Rightarrow> t_conf"
-  where
-  "steps c p 0 = c" |
-  "steps c p (Suc n) = steps (tstep c p) p n"
-
-lemma tstep_red: "steps c p (Suc n) = tstep (steps c p n) p"
-proof(induct n arbitrary: c)
-  fix c
-  show "steps c p (Suc 0) = tstep (steps c p 0) p" by(simp add: steps.simps)
-next
-  fix n c
-  assume ind: "\<And> c. steps c p (Suc n) = tstep (steps c p n) p"
-  have "steps (tstep c p) p (Suc n) = tstep (steps (tstep c p) p n) p"
-    by(rule ind)
-  thus "steps c p (Suc (Suc n)) = tstep (steps c p (Suc n)) p" by(simp add: steps.simps)
-qed
-
-declare Let_def[simp] option.split[split]
-
-definition 
-  "iseven n \<equiv> \<exists> x. n = 2 * x"
-
-
-text {*
-  The following @{text "t_correct"} function is used to specify the wellformedness of Turing
-  machine.
-*}
-fun t_correct :: "tprog \<Rightarrow> bool"
-  where
-  "t_correct p = (length p \<ge> 2 \<and> iseven (length p) \<and> 
-                   list_all (\<lambda> (acn, s). s \<le> length p div 2) p)"
-
-declare t_correct.simps[simp del]
-
-lemma allimp: "\<lbrakk>\<forall>x. P x \<longrightarrow> Q x; \<forall>x. P x\<rbrakk> \<Longrightarrow> \<forall>x. Q x"
-by(auto elim: allE)
-
-lemma halt_lemma: "\<lbrakk>wf LE; \<forall> n. (\<not> P (f n) \<longrightarrow> (f (Suc n), (f n)) \<in> LE)\<rbrakk> \<Longrightarrow> \<exists> n. P (f n)"
-apply(rule exCI, drule allimp, auto)
-apply(drule_tac f = f  in wf_inv_image, simp add: inv_image_def)
-apply(erule wf_induct, auto)
-done
-
-lemma steps_add: "steps c t (x + y) = steps (steps c t x) t y"
-by(induct x arbitrary: c, auto simp: steps.simps tstep_red)
-
-lemma listall_set: "list_all p t \<Longrightarrow> \<forall> a \<in> set t. p a"
-by(induct t, auto)
-
-lemma fetch_ex: "\<exists>b a. fetch T aa ab = (b, a)"
-by(simp add: fetch.simps)
-definition exponent :: "'a \<Rightarrow> nat \<Rightarrow> 'a list" ("_\<^bsup>_\<^esup>" [0, 0]100)
-  where "exponent x n = replicate n x"
-
-text {* 
-  @{text "tinres l1 l2"} means left list @{text "l1"} is congruent with
-  @{text "l2"} with respect to the execution of Turing machine. 
-  Appending Blank to the right of eigther one does not affect the 
-  outcome of excution. 
-*}
-
-definition tinres :: "block list \<Rightarrow> block list \<Rightarrow> bool"
-  where
-  "tinres bx by = (\<exists> n. bx = by@Bk\<^bsup>n\<^esup> \<or> by = bx @ Bk\<^bsup>n\<^esup>)"
-
-lemma exp_zero: "a\<^bsup>0\<^esup> = []"
-by(simp add: exponent_def)
-lemma exp_ind_def: "a\<^bsup>Suc x \<^esup> = a # a\<^bsup>x\<^esup>"
-by(simp add: exponent_def)
-
-text {*
-  The following lemma shows the meaning of @{text "tinres"} with respect to 
-  one step execution.
-  *}
-lemma tinres_step: 
-  "\<lbrakk>tinres l l'; tstep (ss, l, r) t = (sa, la, ra); tstep (ss, l', r) t = (sb, lb, rb)\<rbrakk>
-    \<Longrightarrow> tinres la lb \<and> ra = rb \<and> sa = sb"
-apply(auto simp: tstep.simps fetch.simps new_tape.simps 
-        split: if_splits taction.splits list.splits
-                 block.splits)
-apply(case_tac [!] "t ! (2 * (ss - Suc 0))", 
-     auto simp: exponent_def tinres_def split: if_splits taction.splits list.splits
-                 block.splits)
-apply(case_tac [!] "t ! (2 * (ss - Suc 0) + Suc 0)", 
-     auto simp: exponent_def tinres_def split: if_splits taction.splits list.splits
-                 block.splits)
-done
-
-declare tstep.simps[simp del] steps.simps[simp del]
-
-text {*
-  The following lemma shows the meaning of @{text "tinres"} with respect to 
-  many step execution.
-  *}
-lemma tinres_steps: 
-  "\<lbrakk>tinres l l'; steps (ss, l, r) t stp = (sa, la, ra); steps (ss, l', r) t stp = (sb, lb, rb)\<rbrakk>
-    \<Longrightarrow> tinres la lb \<and> ra = rb \<and> sa = sb"
-apply(induct stp arbitrary: sa la ra sb lb rb, simp add: steps.simps)
-apply(simp add: tstep_red)
-apply(case_tac "(steps (ss, l, r) t stp)")
-apply(case_tac "(steps (ss, l', r) t stp)")
-proof -
-  fix stp sa la ra sb lb rb a b c aa ba ca
-  assume ind: "\<And>sa la ra sb lb rb. \<lbrakk>steps (ss, l, r) t stp = (sa, la, ra); 
-          steps (ss, l', r) t stp = (sb, lb, rb)\<rbrakk> \<Longrightarrow> tinres la lb \<and> ra = rb \<and> sa = sb"
-  and h: " tinres l l'" "tstep (steps (ss, l, r) t stp) t = (sa, la, ra)"
-         "tstep (steps (ss, l', r) t stp) t = (sb, lb, rb)" "steps (ss, l, r) t stp = (a, b, c)" 
-         "steps (ss, l', r) t stp = (aa, ba, ca)"
-  have "tinres b ba \<and> c = ca \<and> a = aa"
-    apply(rule_tac ind, simp_all add: h)
-    done
-  thus "tinres la lb \<and> ra = rb \<and> sa = sb"
-    apply(rule_tac l = b and l' = ba and r = c  and ss = a   
-            and t = t in tinres_step)
-    using h
-    apply(simp, simp, simp)
-    done
-qed
-
-text {*
-  The following function @{text "tshift tp n"} is used to shift Turing programs 
-  @{text "tp"} by @{text "n"} when it is going to be combined with others.
-  *}
-
-fun tshift :: "tprog \<Rightarrow> nat \<Rightarrow> tprog"
-  where
-  "tshift tp off = (map (\<lambda> (action, state). (action, (if state = 0 then 0
-                                                      else state + off))) tp)"
-
-text {*
-  When two Turing programs are combined, the end state (state @{text "0"}) of the one 
-  at the prefix position needs to be connected to the start state 
-  of the one at postfix position. If @{text "tp"} is the Turing program
-  to be at the prefix, @{text "change_termi_state tp"} is the transformed Turing program.
-  *}
-fun change_termi_state :: "tprog \<Rightarrow> tprog"
-  where
-  "change_termi_state t = 
-       (map (\<lambda> (acn, ns). if ns = 0 then (acn, Suc ((length t) div 2)) else (acn, ns)) t)"
-
-text {*
-  @{text "t_add tp1 tp2"} is the combined Truing program.
-*}
-
-fun t_add :: "tprog \<Rightarrow> tprog \<Rightarrow> tprog" ("_ |+| _" [0, 0] 100)
-  where
-  "t_add t1 t2 = ((change_termi_state t1) @ (tshift t2 ((length t1) div 2)))"
-
-text {*
-  Tests whether the current configuration is at state @{text "0"}.
-*}
-definition isS0 :: "t_conf \<Rightarrow> bool"
-  where
-  "isS0 c = (let (s, l, r) = c in s = 0)"
-
-declare tstep.simps[simp del] steps.simps[simp del] 
-        t_add.simps[simp del] fetch.simps[simp del]
-        new_tape.simps[simp del]
-
-
-text {*
-  Single step execution starting from state @{text "0"} will not make any progress.
-*}
-lemma tstep_0: "tstep (0, tp) p = (0, tp)"
-apply(simp add: tstep.simps fetch.simps new_tape.simps)
-done
-
-
-text {*
-  Many step executions starting from state @{text "0"} will not make any progress.
-*}
-
-lemma steps_0: "steps (0, tp) p stp = (0, tp)"
-apply(induct stp)
-apply(simp add: steps.simps)
-apply(simp add: tstep_red tstep_0)
-done
-
-lemma s_keep_step: "\<lbrakk>a \<le> length A div 2; tstep (a, b, c) A = (s, l, r); t_correct A\<rbrakk>
-  \<Longrightarrow> s \<le> length A div 2"
-apply(simp add: tstep.simps fetch.simps t_correct.simps iseven_def 
-  split: if_splits block.splits list.splits)
-apply(case_tac [!] a, auto simp: list_all_length)
-apply(erule_tac x = "2 * nat" in allE, auto)
-apply(erule_tac x = "2 * nat" in allE, auto)
-apply(erule_tac x = "Suc (2 * nat)" in allE, auto)
-done
-
-lemma s_keep: "\<lbrakk>steps (Suc 0, tp) A stp = (s, l, r);  t_correct A\<rbrakk> \<Longrightarrow> s \<le> length A div 2"
-proof(induct stp arbitrary: s l r)
-  case 0 thus "?case" by(auto simp: t_correct.simps steps.simps)
-next
-  fix stp s l r
-  assume ind: "\<And>s l r. \<lbrakk>steps (Suc 0, tp) A stp = (s, l, r); t_correct A\<rbrakk> \<Longrightarrow> s \<le> length A div 2"
-  and h1: "steps (Suc 0, tp) A (Suc stp) = (s, l, r)"
-  and h2: "t_correct A"
-  from h1 h2 show "s \<le> length A div 2"
-  proof(simp add: tstep_red, cases "(steps (Suc 0, tp) A stp)", simp)
-    fix a b c 
-    assume h3: "tstep (a, b, c) A = (s, l, r)"
-    and h4: "steps (Suc 0, tp) A stp = (a, b, c)"
-    have "a \<le> length A div 2"
-      using h2 h4
-      by(rule_tac l = b and r = c in ind, auto)
-    thus "?thesis"
-      using h3 h2
-      by(simp add: s_keep_step)
-  qed
-qed
-
-lemma t_merge_fetch_pre:
-  "\<lbrakk>fetch A s b = (ac, ns); s \<le> length A div 2; t_correct A; s \<noteq> 0\<rbrakk> \<Longrightarrow> 
-  fetch (A |+| B) s b = (ac, if ns = 0 then Suc (length A div 2) else ns)"
-apply(subgoal_tac "2 * (s - Suc 0) < length A \<and> Suc (2 * (s - Suc 0)) < length A")
-apply(auto simp: fetch.simps t_add.simps split: if_splits block.splits)
-apply(simp_all add: nth_append change_termi_state.simps)
-done
-
-lemma [simp]:  "\<lbrakk>\<not> a \<le> length A div 2; t_correct A\<rbrakk> \<Longrightarrow> fetch A a b = (Nop, 0)"
-apply(auto simp: fetch.simps del: nth_of.simps split: block.splits)
-apply(case_tac [!] a, auto simp: t_correct.simps iseven_def)
-done
-
-lemma  [elim]: "\<lbrakk>t_correct A; \<not> isS0 (tstep (a, b, c) A)\<rbrakk> \<Longrightarrow> a \<le> length A div 2"
-apply(rule_tac classical, auto simp: tstep.simps new_tape.simps isS0_def)
-done
-
-lemma [elim]: "\<lbrakk>t_correct A; \<not> isS0 (tstep (a, b, c) A)\<rbrakk> \<Longrightarrow> 0 < a"
-apply(rule_tac classical, simp add: tstep_0 isS0_def)
-done
-
-
-lemma t_merge_pre_eq_step: "\<lbrakk>tstep (a, b, c) A = cf; t_correct A; \<not> isS0 cf\<rbrakk> 
-        \<Longrightarrow> tstep (a, b, c) (A |+| B) = cf"
-apply(subgoal_tac "a \<le> length A div 2 \<and> a \<noteq> 0")
-apply(simp add: tstep.simps)
-apply(case_tac "fetch A a (case c of [] \<Rightarrow> Bk | x # xs \<Rightarrow> x)", simp)
-apply(drule_tac B = B in t_merge_fetch_pre, simp, simp, simp, simp add: isS0_def, auto)
-done
-
-lemma t_merge_pre_eq:  "\<lbrakk>steps (Suc 0, tp) A stp = cf; \<not> isS0 cf; t_correct A\<rbrakk>
-    \<Longrightarrow> steps (Suc 0, tp) (A |+| B) stp = cf"
-proof(induct stp arbitrary: cf)
-  case 0 thus "?case" by(simp add: steps.simps)
-next
-  fix stp cf
-  assume ind: "\<And>cf. \<lbrakk>steps (Suc 0, tp) A stp = cf; \<not> isS0 cf; t_correct A\<rbrakk> 
-                 \<Longrightarrow> steps (Suc 0, tp) (A |+| B) stp = cf"
-  and h1: "steps (Suc 0, tp) A (Suc stp) = cf"
-  and h2: "\<not> isS0 cf"
-  and h3: "t_correct A"
-  from h1 h2 h3 show "steps (Suc 0, tp) (A |+| B) (Suc stp) = cf"
-  proof(simp add: tstep_red, cases "steps (Suc 0, tp) (A) stp", simp)
-    fix a b c
-    assume h4: "tstep (a, b, c) A = cf"
-    and h5: "steps (Suc 0, tp) A stp = (a, b, c)"
-    have "steps (Suc 0, tp) (A |+| B) stp = (a, b, c)"
-    proof(cases a)
-      case 0 thus "?thesis"
-        using h4 h2
-        apply(simp add: tstep_0, cases cf, simp add: isS0_def)
-        done
-    next
-      case (Suc n) thus "?thesis"
-        using h5 h3
-        apply(rule_tac ind, auto simp: isS0_def)
-        done
-    qed
-    thus "tstep (steps (Suc 0, tp) (A |+| B) stp) (A |+| B) = cf"
-      using h4 h5 h3 h2
-      apply(simp)
-      apply(rule t_merge_pre_eq_step, auto)
-      done
-  qed
-qed
-
-declare nth.simps[simp del] tshift.simps[simp del] change_termi_state.simps[simp del]
-
-lemma [simp]: "length (change_termi_state A) = length A"
-by(simp add: change_termi_state.simps)
-
-lemma first_halt_point: "steps (Suc 0, tp) A stp = (0, tp')
- \<Longrightarrow> \<exists>stp. \<not> isS0 (steps (Suc 0, tp) A stp) \<and> steps (Suc 0, tp) A (Suc stp) = (0, tp')"
-proof(induct stp)
-  case 0  thus "?case" by(simp add: steps.simps)
-next
-  case (Suc n) 
-  fix stp
-  assume ind: "steps (Suc 0, tp) A stp = (0, tp') \<Longrightarrow> 
-       \<exists>stp. \<not> isS0 (steps (Suc 0, tp) A stp) \<and> steps (Suc 0, tp) A (Suc stp) = (0, tp')"
-    and h: "steps (Suc 0, tp) A (Suc stp) = (0, tp')"
-  from h show "\<exists>stp. \<not> isS0 (steps (Suc 0, tp) A stp) \<and> steps (Suc 0, tp) A (Suc stp) = (0, tp')"
-  proof(simp add: tstep_red, cases "steps (Suc 0, tp) A stp", simp, case_tac a)
-    fix a b c
-    assume g1: "a = (0::nat)"
-    and g2: "tstep (a, b, c) A = (0, tp')"
-    and g3: "steps (Suc 0, tp) A stp = (a, b, c)"
-    have "steps (Suc 0, tp) A stp = (0, tp')"
-      using g2 g1 g3
-      by(simp add: tstep_0)
-    hence "\<exists> stp. \<not> isS0 (steps (Suc 0, tp) A stp) \<and> steps (Suc 0, tp) A (Suc stp) = (0, tp')"
-      by(rule ind)
-    thus "\<exists>stp. \<not> isS0 (steps (Suc 0, tp) A stp) \<and> tstep (steps (Suc 0, tp) A stp) A = (0, tp')" 
-      apply(simp add: tstep_red)
-      done
-  next
-    fix a b c nat
-    assume g1: "steps (Suc 0, tp) A stp = (a, b, c)"
-    and g2: "steps (Suc 0, tp) A (Suc stp) = (0, tp')" "a= Suc nat"
-    thus "\<exists>stp. \<not> isS0 (steps (Suc 0, tp) A stp) \<and> tstep (steps (Suc 0, tp) A stp) A = (0, tp')"
-      apply(rule_tac x = stp in exI)
-      apply(simp add: isS0_def tstep_red)
-      done
-  qed
-qed 
-   
-lemma t_merge_pre_halt_same': 
-  "\<lbrakk>\<not> isS0 (steps (Suc 0, tp) A stp) ; steps (Suc 0, tp) A (Suc stp) = (0, tp'); t_correct A\<rbrakk>
-  \<Longrightarrow> steps (Suc 0, tp) (A |+| B) (Suc stp) = (Suc (length A div 2), tp')"    
-proof(simp add: tstep_red, cases "steps (Suc 0, tp) A stp", simp)
-  fix a b c 
-  assume h1: "\<not> isS0 (a, b, c)"
-  and h2: "tstep (a, b, c) A = (0, tp')"
-  and h3: "t_correct A"
-  and h4: "steps (Suc 0, tp) A stp = (a, b, c)"
-  have "steps (Suc 0, tp) (A |+| B) stp = (a, b, c)"
-    using h1 h4 h3
-    apply(rule_tac  t_merge_pre_eq, auto)
-    done
-  moreover have "tstep (a, b, c) (A |+| B) = (Suc (length A div 2), tp')"
-    using h2 h3 h1 h4 
-    apply(simp add: tstep.simps)
-    apply(case_tac " fetch A a (case c of [] \<Rightarrow> Bk | x # xs \<Rightarrow> x)", simp)
-    apply(drule_tac B = B in t_merge_fetch_pre, auto simp: isS0_def intro: s_keep)
-    done
-  ultimately show "tstep (steps (Suc 0, tp) (A |+| B) stp) (A |+| B) = (Suc (length A div 2), tp')"
-    by(simp)
-qed
-
-text {*
-  When Turing machine @{text "A"} and @{text "B"} are combined and the execution
-  of @{text "A"} can termination within @{text "stp"} steps, 
-  the combined machine @{text "A |+| B"} will eventually get into the starting 
-  state of machine @{text "B"}.
-*}
-lemma t_merge_pre_halt_same: "
-  \<lbrakk>steps (Suc 0, tp) A stp = (0, tp'); t_correct A; t_correct B\<rbrakk>
-     \<Longrightarrow> \<exists> stp. steps (Suc 0, tp) (A |+| B) stp = (Suc (length A div 2), tp')"
-proof -
-  assume a_wf: "t_correct A"
-  and b_wf: "t_correct B"
-  and a_ht: "steps (Suc 0, tp) A stp = (0, tp')"
-  have halt_point: "\<exists> stp. \<not> isS0 (steps (Suc 0, tp) A stp) \<and> steps (Suc 0, tp) A (Suc stp) = (0, tp')"
-    using a_ht
-    by(erule_tac first_halt_point)
-  then obtain stp' where "\<not> isS0 (steps (Suc 0, tp) A stp') \<and> steps (Suc 0, tp) A (Suc stp') = (0, tp')"..
-  hence "steps (Suc 0, tp) (A |+| B) (Suc stp') = (Suc (length A div 2), tp')"
-    using a_wf
-    apply(rule_tac t_merge_pre_halt_same', auto)
-    done
-  thus "?thesis" ..
-qed
-
-lemma fetch_0: "fetch p 0 b = (Nop, 0)"
-by(simp add: fetch.simps)
-
-lemma [simp]: "length (tshift B x) = length B"
-by(simp add: tshift.simps)
-
-lemma [simp]: "t_correct A \<Longrightarrow> 2 * (length A div 2) = length A"
-apply(simp add: t_correct.simps iseven_def, auto)
-done
-
-lemma t_merge_fetch_snd: 
-  "\<lbrakk>fetch B a b = (ac, ns); t_correct A; t_correct B; a > 0 \<rbrakk>
-  \<Longrightarrow> fetch (A |+| B) (a + length A div 2) b
-  = (ac, if ns = 0 then 0 else ns + length A div 2)"
-apply(auto simp: fetch.simps t_add.simps split: if_splits block.splits)
-apply(case_tac [!] a, simp_all)
-apply(simp_all add: nth_append change_termi_state.simps tshift.simps)
-done
-
-lemma t_merge_snd_eq_step: 
-  "\<lbrakk>tstep (s, l, r) B = (s', l', r'); t_correct A; t_correct B; s > 0\<rbrakk>
-    \<Longrightarrow> tstep (s + length A div 2, l, r) (A |+| B) = 
-       (if s' = 0 then 0 else s' + length A div 2, l' ,r') "
-apply(simp add: tstep.simps)
-apply(cases "fetch B s (case r of [] \<Rightarrow> Bk | x # xs \<Rightarrow> x)")
-apply(auto simp: t_merge_fetch_snd)
-apply(frule_tac [!] t_merge_fetch_snd, auto)
-done 
-
-text {*
-  Relates the executions of TM @{text "B"}, one is when @{text "B"} is executed alone,
-  the other is the execution when @{text "B"} is in the combined TM.
-*}
-lemma t_merge_snd_eq_steps: 
-  "\<lbrakk>t_correct A; t_correct B; steps (s, l, r) B stp = (s', l', r'); s > 0\<rbrakk>
-  \<Longrightarrow> steps (s + length A div 2, l, r) (A |+| B) stp = 
-      (if s' = 0 then 0 else s' + length A div 2, l', r')"
-proof(induct stp arbitrary: s' l' r')
-  case 0 thus "?case" 
-    by(simp add: steps.simps)
-next
-  fix stp s' l' r'
-  assume ind: "\<And>s' l' r'. \<lbrakk>t_correct A; t_correct B; steps (s, l, r) B stp = (s', l', r'); 0 < s\<rbrakk>
-                   \<Longrightarrow> steps (s + length A div 2, l, r) (A |+| B) stp = 
-                          (if s' = 0 then 0 else s' + length A div 2, l', r')"
-  and h1: "steps (s, l, r) B (Suc stp) = (s', l', r')"
-  and h2: "t_correct A"
-  and h3: "t_correct B"
-  and h4: "0 < s"
-  from h1 show "steps (s + length A div 2, l, r) (A |+| B) (Suc stp) 
-            = (if s' = 0 then 0 else s' + length A div 2, l', r')"
-  proof(simp only: tstep_red, cases "steps (s, l, r) B stp")
-    fix a b c 
-    assume h5: "steps (s, l, r) B stp = (a, b, c)" "tstep (steps (s, l, r) B stp) B = (s', l', r')"
-    hence h6: "(steps (s + length A div 2, l, r) (A |+| B) stp) = 
-                ((if a = 0 then 0 else a + length A div 2, b, c))"
-      using h2 h3 h4
-      by(rule_tac ind, auto)
-    thus "tstep (steps (s + length A div 2, l, r) (A |+| B) stp) (A |+| B) = 
-       (if s' = 0 then 0 else s'+ length A div 2, l', r')"
-      using h5
-    proof(auto)
-      assume "tstep (0, b, c) B = (0, l', r')" thus "tstep (0, b, c) (A |+| B) = (0, l', r')"
-        by(simp add: tstep_0)
-    next
-      assume "tstep (0, b, c) B = (s', l', r')" "0 < s'"
-      thus "tstep (0, b, c) (A |+| B) = (s' + length A div 2, l', r')"
-        by(simp add: tstep_0)
-    next
-      assume "tstep (a, b, c) B = (0, l', r')" "0 < a"
-      thus "tstep (a + length A div 2, b, c) (A |+| B) = (0, l', r')"
-        using h2 h3
-        by(drule_tac t_merge_snd_eq_step, auto)
-    next
-      assume "tstep (a, b, c) B = (s', l', r')" "0 < a" "0 < s'"
-      thus "tstep (a + length A div 2, b, c) (A |+| B) = (s' + length A div 2, l', r')"
-        using h2 h3
-        by(drule_tac t_merge_snd_eq_step, auto)
-    qed
-  qed
-qed
-
-lemma t_merge_snd_halt_eq: 
-  "\<lbrakk>steps (Suc 0, tp) B stp = (0, tp'); t_correct A; t_correct B\<rbrakk>
-  \<Longrightarrow> \<exists>stp. steps (Suc (length A div 2), tp) (A |+| B) stp = (0, tp')"
-apply(case_tac tp, cases tp', simp)
-apply(drule_tac  s = "Suc 0" in t_merge_snd_eq_steps, auto)
-done
-
-lemma t_inj: "\<lbrakk>steps (Suc 0, tp) A stpa = (0, tp1); steps (Suc 0, tp) A stpb = (0, tp2)\<rbrakk> 
-      \<Longrightarrow> tp1 = tp2"
-proof -
-  assume h1: "steps (Suc 0, tp) A stpa = (0, tp1)" 
-  and h2: "steps (Suc 0, tp) A stpb = (0, tp2)"
-  thus "?thesis"
-  proof(cases "stpa < stpb")
-    case True thus "?thesis"
-      using h1 h2
-      apply(drule_tac less_imp_Suc_add, auto)
-      apply(simp del: add_Suc_right add_Suc add: add_Suc_right[THEN sym] steps_add steps_0)
-      done
-  next
-    case False thus "?thesis"
-      using h1 h2
-      apply(drule_tac leI)
-      apply(case_tac "stpb = stpa", auto)
-      apply(subgoal_tac "stpb < stpa")
-      apply(drule_tac less_imp_Suc_add, auto)
-      apply(simp del: add_Suc_right add_Suc add: add_Suc_right[THEN sym] steps_add steps_0)
-      done
-  qed
-qed
-
-type_synonym t_assert = "tape \<Rightarrow> bool"
-
-definition t_imply :: "t_assert \<Rightarrow> t_assert \<Rightarrow> bool" ("_ \<turnstile>-> _" [0, 0] 100)
-  where
-  "t_imply a1 a2 = (\<forall> tp. a1 tp \<longrightarrow> a2 tp)"
-
-
-locale turing_merge =
-  fixes A :: "tprog" and B :: "tprog" and P1 :: "t_assert"
-  and P2 :: "t_assert"
-  and P3 :: "t_assert"
-  and P4 :: "t_assert"
-  and Q1:: "t_assert"
-  and Q2 :: "t_assert"
-  assumes 
-  A_wf : "t_correct A"
-  and B_wf : "t_correct B"
-  and A_halt : "P1 tp \<Longrightarrow> \<exists> stp. let (s, tp') = steps (Suc 0, tp) A stp in s = 0 \<and> Q1 tp'"
-  and B_halt : "P2 tp \<Longrightarrow> \<exists> stp. let (s, tp') = steps (Suc 0, tp) B stp in s = 0 \<and> Q2 tp'"
-  and A_uhalt : "P3 tp \<Longrightarrow> \<not> (\<exists> stp. isS0 (steps (Suc 0, tp) A stp))"
-  and B_uhalt: "P4 tp \<Longrightarrow> \<not> (\<exists> stp. isS0 (steps (Suc 0, tp) B stp))"
-begin
-
-
-text {*
-  The following lemma tries to derive the Hoare logic rule for sequentially combined TMs.
-  It deals with the situtation when both @{text "A"} and @{text "B"} are terminated.
-*}
-
-lemma t_merge_halt: 
-  assumes aimpb: "Q1 \<turnstile>-> P2"
-  shows "P1 \<turnstile>->  \<lambda> tp. (\<exists> stp tp'. steps (Suc 0, tp) (A |+| B)  stp = (0, tp') \<and> Q2 tp')"
-proof(simp add: t_imply_def, auto)
-  fix a b
-  assume h: "P1 (a, b)"
-  hence "\<exists> stp. let (s, tp') = steps (Suc 0, a, b) A stp in s = 0 \<and> Q1 tp'"
-    using A_halt by simp
-  from this obtain stp1 where "let (s, tp') = steps (Suc 0, a, b) A stp1 in s = 0 \<and> Q1 tp'" ..
-  thus "\<exists>stp aa ba. steps (Suc 0, a, b) (A |+| B) stp = (0, aa, ba) \<and> Q2 (aa, ba)"
-  proof(case_tac "steps (Suc 0, a, b) A stp1", simp, erule_tac conjE)
-    fix aa ba c
-    assume g1: "Q1 (ba, c)" 
-      and g2: "steps (Suc 0, a, b) A stp1 = (0, ba, c)"
-    hence "P2 (ba, c)"
-      using aimpb apply(simp add: t_imply_def)
-      done
-    hence "\<exists> stp. let (s, tp') = steps (Suc 0, ba, c) B stp in s = 0 \<and> Q2 tp'"
-      using B_halt by simp
-    from this obtain stp2 where "let (s, tp') = steps (Suc 0, ba, c) B stp2 in s = 0 \<and> Q2 tp'" ..
-    thus "?thesis"
-    proof(case_tac "steps (Suc 0, ba, c) B stp2", simp, erule_tac conjE)
-      fix aa bb ca
-      assume g3: " Q2 (bb, ca)" "steps (Suc 0, ba, c) B stp2 = (0, bb, ca)"
-      have "\<exists> stp. steps (Suc 0, a, b) (A |+| B) stp = (Suc (length A div 2), ba , c)"
-        using g2 A_wf B_wf
-        by(rule_tac t_merge_pre_halt_same, auto)
-      moreover have "\<exists> stp. steps (Suc (length A div 2), ba, c) (A |+| B) stp = (0, bb, ca)"
-        using g3 A_wf B_wf
-        apply(rule_tac t_merge_snd_halt_eq, auto)
-        done
-      ultimately show "\<exists>stp aa ba. steps (Suc 0, a, b) (A |+| B) stp = (0, aa, ba) \<and> Q2 (aa, ba)"
-        apply(erule_tac exE, erule_tac exE)
-        apply(rule_tac x = "stp + stpa" in exI, simp add: steps_add)
-        using g3 by simp
-    qed
-  qed
-qed
-
-lemma  t_merge_uhalt_tmp:
-  assumes B_uh: "\<forall>stp. \<not> isS0 (steps (Suc 0, b, c) B stp)"
-  and merge_ah: "steps (Suc 0, tp) (A |+| B) stpa = (Suc (length A div 2), b, c)" 
-  shows "\<forall> stp. \<not> isS0 (steps (Suc 0, tp) (A |+| B) stp)"
-  using B_uh merge_ah
-apply(rule_tac allI)
-apply(case_tac "stp > stpa")
-apply(erule_tac x = "stp - stpa" in allE)
-apply(case_tac "(steps (Suc 0, b, c) B (stp - stpa))", simp)
-proof -
-  fix stp a ba ca 
-  assume h1: "\<not> isS0 (a, ba, ca)" "stpa < stp"
-  and h2: "steps (Suc 0, b, c) B (stp - stpa) = (a, ba, ca)"
-  have "steps (Suc 0 + length A div 2, b, c) (A |+| B) (stp - stpa) = 
-      (if a = 0 then 0 else a + length A div 2, ba, ca)"
-    using A_wf B_wf h2
-    by(rule_tac t_merge_snd_eq_steps, auto)
-  moreover have "a > 0" using h1 by(simp add: isS0_def)
-  moreover have "\<exists> stpb. stp = stpa + stpb" 
-    using h1 by(rule_tac x = "stp - stpa" in exI, simp)
-  ultimately show "\<not> isS0 (steps (Suc 0, tp) (A |+| B) stp)"
-    using merge_ah
-    by(auto simp: steps_add isS0_def)
-next
-  fix stp
-  assume h: "steps (Suc 0, tp) (A |+| B) stpa = (Suc (length A div 2), b, c)" "\<not> stpa < stp"
-  hence "\<exists> stpb. stpa = stp + stpb" apply(rule_tac x = "stpa - stp" in exI, auto) done
-  thus "\<not> isS0 (steps (Suc 0, tp) (A |+| B) stp)"
-    using h
-    apply(auto)
-    apply(cases "steps (Suc 0, tp) (A |+| B) stp", simp add: steps_add isS0_def steps_0)
-    done
-qed
-
-text {*
-  The following lemma deals with the situation when TM @{text "B"} can not terminate.
-  *}
-
-lemma t_merge_uhalt: 
-  assumes aimpb: "Q1 \<turnstile>-> P4"
-  shows "P1 \<turnstile>-> \<lambda> tp. \<not> (\<exists> stp. isS0 (steps (Suc 0, tp) (A |+| B) stp))"
-proof(simp only: t_imply_def, rule_tac allI, rule_tac impI)
-  fix tp 
-  assume init_asst: "P1 tp"
-  show "\<not> (\<exists>stp. isS0 (steps (Suc 0, tp) (A |+| B) stp))"
-  proof -
-    have "\<exists> stp. let (s, tp') = steps (Suc 0, tp) A stp in s = 0 \<and> Q1 tp'"
-      using A_halt[of tp] init_asst
-      by(simp)
-    from this obtain stpx where "let (s, tp') = steps (Suc 0, tp) A stpx in s = 0 \<and> Q1 tp'" ..
-    thus "?thesis"
-    proof(cases "steps (Suc 0, tp) A stpx", simp, erule_tac conjE)
-      fix a b c
-      assume "Q1 (b, c)"
-        and h3: "steps (Suc 0, tp) A stpx = (0, b, c)"
-      hence h2: "P4 (b, c)"  using aimpb
-        by(simp add: t_imply_def)
-      have "\<exists> stp. steps (Suc 0, tp) (A |+| B) stp = (Suc (length A div 2), b, c)"
-        using h3 A_wf B_wf
-        apply(rule_tac stp = stpx in t_merge_pre_halt_same, auto)
-        done
-      from this obtain stpa where h4:"steps (Suc 0, tp) (A |+| B) stpa = (Suc (length A div 2), b, c)" ..
-      have " \<not> (\<exists> stp. isS0 (steps (Suc 0, b, c) B stp))"
-        using B_uhalt [of "(b, c)"] h2 apply simp
-        done
-      from this and h4 show "\<forall>stp. \<not> isS0 (steps (Suc 0, tp) (A |+| B) stp)"
-        by(rule_tac t_merge_uhalt_tmp, auto)
-    qed
-  qed
-qed
-end
-
-end
-
+theory turing_basic
+imports Main
+begin
+
+section {* Basic definitions of Turing machine *}
+
+(* Title: Turing machine's definition and its charater
+   Author: Xu Jian <xujian817@hotmail.com>
+   Maintainer: Xu Jian
+*)
+
+text {*
+  Actions of Turing machine (Abbreviated TM in the following* ).
+*}
+
+datatype taction = 
+  -- {* Write zero *}
+  W0 | 
+  -- {* Write one *}
+  W1 | 
+  -- {* Move left *}
+  L | 
+  -- {* Move right *}
+  R | 
+  -- {* Do nothing *}
+  Nop
+
+text {*
+  Tape contents in every block.
+*}
+
+datatype block = 
+  -- {* Blank *}
+  Bk | 
+  -- {* Occupied *}
+  Oc
+
+text {*
+  Tape is represented as a pair of lists $(L_{left}, L_{right})$,
+  where $L_left$, named {\em left list}, is used to represent
+  the tape to the left of RW-head and
+  $L_{right}$, named {\em right list}, is used to represent the tape
+  under and to the right of RW-head.
+*}
+
+type_synonym tape = "block list \<times> block list"
+
+text {* The state of turing machine.*}
+type_synonym tstate = nat
+
+text {*
+  Turing machine instruction is represented as a 
+  pair @{text "(action, next_state)"},
+  where @{text "action"} is the action to take at the current state 
+  and @{text "next_state"} is the next state the machine is getting into
+  after the action.
+*}
+type_synonym tinst = "taction \<times> tstate"
+
+text {*
+  Program of Turing machine is represented as a list of Turing instructions
+  and the execution of the program starts from the head of the list.
+  *}
+type_synonym tprog = "tinst list"
+
+
+text {*
+  Turing machine configuration, which consists of the current state 
+  and the tape.
+*}
+type_synonym t_conf = "tstate \<times> tape"
+
+fun nth_of ::  "'a list \<Rightarrow> nat \<Rightarrow> 'a option"
+  where
+  "nth_of xs n = (if n < length xs then Some (xs!n)
+                  else None)"
+
+text {*
+  The function used to fetech instruction out of Turing program.
+  *}
+
+fun fetch :: "tprog \<Rightarrow> tstate \<Rightarrow> block \<Rightarrow> tinst"
+  where
+  "fetch p s b = (if s = 0 then (Nop, 0) else
+                  case b of 
+                     Bk \<Rightarrow> case nth_of p (2 * (s - 1)) of
+                          Some i \<Rightarrow> i
+                        | None \<Rightarrow> (Nop, 0) 
+                   | Oc \<Rightarrow> case nth_of p (2 * (s - 1) +1) of
+                          Some i \<Rightarrow> i
+                        | None \<Rightarrow> (Nop, 0))"
+
+
+fun new_tape :: "taction \<Rightarrow> tape \<Rightarrow> tape"
+where 
+   "new_tape action (leftn, rightn) = (case action of
+                                         W0 \<Rightarrow> (leftn, Bk#(tl rightn)) |
+                                         W1 \<Rightarrow> (leftn, Oc#(tl rightn)) |
+                                         L  \<Rightarrow>  (if leftn = [] then (tl leftn, Bk#rightn)
+                                               else (tl leftn, (hd leftn) # rightn)) |
+                                         R  \<Rightarrow> if rightn = [] then (Bk#leftn,tl rightn) 
+                                               else ((hd rightn)#leftn, tl rightn) |
+                                         Nop \<Rightarrow> (leftn, rightn)
+                                       )"
+
+text {*
+  The one step function used to transfer Turing machine configuration.
+*}
+fun tstep :: "t_conf \<Rightarrow> tprog \<Rightarrow> t_conf"
+  where
+  "tstep c p = (let (s, l, r) = c in 
+                     let (ac, ns) = (fetch p s (case r of [] \<Rightarrow> Bk |     
+                                                               x # xs \<Rightarrow> x)) in
+                       (ns, new_tape ac (l, r)))"
+
+text {*
+  The many-step function.
+*}
+fun steps :: "t_conf \<Rightarrow> tprog \<Rightarrow> nat \<Rightarrow> t_conf"
+  where
+  "steps c p 0 = c" |
+  "steps c p (Suc n) = steps (tstep c p) p n"
+
+lemma tstep_red: "steps c p (Suc n) = tstep (steps c p n) p"
+proof(induct n arbitrary: c)
+  fix c
+  show "steps c p (Suc 0) = tstep (steps c p 0) p" by(simp add: steps.simps)
+next
+  fix n c
+  assume ind: "\<And> c. steps c p (Suc n) = tstep (steps c p n) p"
+  have "steps (tstep c p) p (Suc n) = tstep (steps (tstep c p) p n) p"
+    by(rule ind)
+  thus "steps c p (Suc (Suc n)) = tstep (steps c p (Suc n)) p" by(simp add: steps.simps)
+qed
+
+declare Let_def[simp] option.split[split]
+
+definition 
+  "iseven n \<equiv> \<exists> x. n = 2 * x"
+
+
+text {*
+  The following @{text "t_correct"} function is used to specify the wellformedness of Turing
+  machine.
+*}
+fun t_correct :: "tprog \<Rightarrow> bool"
+  where
+  "t_correct p = (length p \<ge> 2 \<and> iseven (length p) \<and> 
+                   list_all (\<lambda> (acn, s). s \<le> length p div 2) p)"
+
+declare t_correct.simps[simp del]
+
+lemma allimp: "\<lbrakk>\<forall>x. P x \<longrightarrow> Q x; \<forall>x. P x\<rbrakk> \<Longrightarrow> \<forall>x. Q x"
+by(auto elim: allE)
+
+lemma halt_lemma: "\<lbrakk>wf LE; \<forall> n. (\<not> P (f n) \<longrightarrow> (f (Suc n), (f n)) \<in> LE)\<rbrakk> \<Longrightarrow> \<exists> n. P (f n)"
+apply(rule exCI, drule allimp, auto)
+apply(drule_tac f = f  in wf_inv_image, simp add: inv_image_def)
+apply(erule wf_induct, auto)
+done
+
+lemma steps_add: "steps c t (x + y) = steps (steps c t x) t y"
+by(induct x arbitrary: c, auto simp: steps.simps tstep_red)
+
+lemma listall_set: "list_all p t \<Longrightarrow> \<forall> a \<in> set t. p a"
+by(induct t, auto)
+
+lemma fetch_ex: "\<exists>b a. fetch T aa ab = (b, a)"
+by(simp add: fetch.simps)
+definition exponent :: "'a \<Rightarrow> nat \<Rightarrow> 'a list" ("_\<^bsup>_\<^esup>" [0, 0]100)
+  where "exponent x n = replicate n x"
+
+text {* 
+  @{text "tinres l1 l2"} means left list @{text "l1"} is congruent with
+  @{text "l2"} with respect to the execution of Turing machine. 
+  Appending Blank to the right of eigther one does not affect the 
+  outcome of excution. 
+*}
+
+definition tinres :: "block list \<Rightarrow> block list \<Rightarrow> bool"
+  where
+  "tinres bx by = (\<exists> n. bx = by@Bk\<^bsup>n\<^esup> \<or> by = bx @ Bk\<^bsup>n\<^esup>)"
+
+lemma exp_zero: "a\<^bsup>0\<^esup> = []"
+by(simp add: exponent_def)
+lemma exp_ind_def: "a\<^bsup>Suc x \<^esup> = a # a\<^bsup>x\<^esup>"
+by(simp add: exponent_def)
+
+text {*
+  The following lemma shows the meaning of @{text "tinres"} with respect to 
+  one step execution.
+  *}
+lemma tinres_step: 
+  "\<lbrakk>tinres l l'; tstep (ss, l, r) t = (sa, la, ra); tstep (ss, l', r) t = (sb, lb, rb)\<rbrakk>
+    \<Longrightarrow> tinres la lb \<and> ra = rb \<and> sa = sb"
+apply(auto simp: tstep.simps fetch.simps new_tape.simps 
+        split: if_splits taction.splits list.splits
+                 block.splits)
+apply(case_tac [!] "t ! (2 * (ss - Suc 0))", 
+     auto simp: exponent_def tinres_def split: if_splits taction.splits list.splits
+                 block.splits)
+apply(case_tac [!] "t ! (2 * (ss - Suc 0) + Suc 0)", 
+     auto simp: exponent_def tinres_def split: if_splits taction.splits list.splits
+                 block.splits)
+done
+
+declare tstep.simps[simp del] steps.simps[simp del]
+
+text {*
+  The following lemma shows the meaning of @{text "tinres"} with respect to 
+  many step execution.
+  *}
+lemma tinres_steps: 
+  "\<lbrakk>tinres l l'; steps (ss, l, r) t stp = (sa, la, ra); steps (ss, l', r) t stp = (sb, lb, rb)\<rbrakk>
+    \<Longrightarrow> tinres la lb \<and> ra = rb \<and> sa = sb"
+apply(induct stp arbitrary: sa la ra sb lb rb, simp add: steps.simps)
+apply(simp add: tstep_red)
+apply(case_tac "(steps (ss, l, r) t stp)")
+apply(case_tac "(steps (ss, l', r) t stp)")
+proof -
+  fix stp sa la ra sb lb rb a b c aa ba ca
+  assume ind: "\<And>sa la ra sb lb rb. \<lbrakk>steps (ss, l, r) t stp = (sa, la, ra); 
+          steps (ss, l', r) t stp = (sb, lb, rb)\<rbrakk> \<Longrightarrow> tinres la lb \<and> ra = rb \<and> sa = sb"
+  and h: " tinres l l'" "tstep (steps (ss, l, r) t stp) t = (sa, la, ra)"
+         "tstep (steps (ss, l', r) t stp) t = (sb, lb, rb)" "steps (ss, l, r) t stp = (a, b, c)" 
+         "steps (ss, l', r) t stp = (aa, ba, ca)"
+  have "tinres b ba \<and> c = ca \<and> a = aa"
+    apply(rule_tac ind, simp_all add: h)
+    done
+  thus "tinres la lb \<and> ra = rb \<and> sa = sb"
+    apply(rule_tac l = b and l' = ba and r = c  and ss = a   
+            and t = t in tinres_step)
+    using h
+    apply(simp, simp, simp)
+    done
+qed
+
+text {*
+  The following function @{text "tshift tp n"} is used to shift Turing programs 
+  @{text "tp"} by @{text "n"} when it is going to be combined with others.
+  *}
+
+fun tshift :: "tprog \<Rightarrow> nat \<Rightarrow> tprog"
+  where
+  "tshift tp off = (map (\<lambda> (action, state). (action, (if state = 0 then 0
+                                                      else state + off))) tp)"
+
+text {*
+  When two Turing programs are combined, the end state (state @{text "0"}) of the one 
+  at the prefix position needs to be connected to the start state 
+  of the one at postfix position. If @{text "tp"} is the Turing program
+  to be at the prefix, @{text "change_termi_state tp"} is the transformed Turing program.
+  *}
+fun change_termi_state :: "tprog \<Rightarrow> tprog"
+  where
+  "change_termi_state t = 
+       (map (\<lambda> (acn, ns). if ns = 0 then (acn, Suc ((length t) div 2)) else (acn, ns)) t)"
+
+text {*
+  @{text "t_add tp1 tp2"} is the combined Truing program.
+*}
+
+fun t_add :: "tprog \<Rightarrow> tprog \<Rightarrow> tprog" ("_ |+| _" [0, 0] 100)
+  where
+  "t_add t1 t2 = ((change_termi_state t1) @ (tshift t2 ((length t1) div 2)))"
+
+text {*
+  Tests whether the current configuration is at state @{text "0"}.
+*}
+definition isS0 :: "t_conf \<Rightarrow> bool"
+  where
+  "isS0 c = (let (s, l, r) = c in s = 0)"
+
+declare tstep.simps[simp del] steps.simps[simp del] 
+        t_add.simps[simp del] fetch.simps[simp del]
+        new_tape.simps[simp del]
+
+
+text {*
+  Single step execution starting from state @{text "0"} will not make any progress.
+*}
+lemma tstep_0: "tstep (0, tp) p = (0, tp)"
+apply(simp add: tstep.simps fetch.simps new_tape.simps)
+done
+
+
+text {*
+  Many step executions starting from state @{text "0"} will not make any progress.
+*}
+
+lemma steps_0: "steps (0, tp) p stp = (0, tp)"
+apply(induct stp)
+apply(simp add: steps.simps)
+apply(simp add: tstep_red tstep_0)
+done
+
+lemma s_keep_step: "\<lbrakk>a \<le> length A div 2; tstep (a, b, c) A = (s, l, r); t_correct A\<rbrakk>
+  \<Longrightarrow> s \<le> length A div 2"
+apply(simp add: tstep.simps fetch.simps t_correct.simps iseven_def 
+  split: if_splits block.splits list.splits)
+apply(case_tac [!] a, auto simp: list_all_length)
+apply(erule_tac x = "2 * nat" in allE, auto)
+apply(erule_tac x = "2 * nat" in allE, auto)
+apply(erule_tac x = "Suc (2 * nat)" in allE, auto)
+done
+
+lemma s_keep: "\<lbrakk>steps (Suc 0, tp) A stp = (s, l, r);  t_correct A\<rbrakk> \<Longrightarrow> s \<le> length A div 2"
+proof(induct stp arbitrary: s l r)
+  case 0 thus "?case" by(auto simp: t_correct.simps steps.simps)
+next
+  fix stp s l r
+  assume ind: "\<And>s l r. \<lbrakk>steps (Suc 0, tp) A stp = (s, l, r); t_correct A\<rbrakk> \<Longrightarrow> s \<le> length A div 2"
+  and h1: "steps (Suc 0, tp) A (Suc stp) = (s, l, r)"
+  and h2: "t_correct A"
+  from h1 h2 show "s \<le> length A div 2"
+  proof(simp add: tstep_red, cases "(steps (Suc 0, tp) A stp)", simp)
+    fix a b c 
+    assume h3: "tstep (a, b, c) A = (s, l, r)"
+    and h4: "steps (Suc 0, tp) A stp = (a, b, c)"
+    have "a \<le> length A div 2"
+      using h2 h4
+      by(rule_tac l = b and r = c in ind, auto)
+    thus "?thesis"
+      using h3 h2
+      by(simp add: s_keep_step)
+  qed
+qed
+
+lemma t_merge_fetch_pre:
+  "\<lbrakk>fetch A s b = (ac, ns); s \<le> length A div 2; t_correct A; s \<noteq> 0\<rbrakk> \<Longrightarrow> 
+  fetch (A |+| B) s b = (ac, if ns = 0 then Suc (length A div 2) else ns)"
+apply(subgoal_tac "2 * (s - Suc 0) < length A \<and> Suc (2 * (s - Suc 0)) < length A")
+apply(auto simp: fetch.simps t_add.simps split: if_splits block.splits)
+apply(simp_all add: nth_append change_termi_state.simps)
+done
+
+lemma [simp]:  "\<lbrakk>\<not> a \<le> length A div 2; t_correct A\<rbrakk> \<Longrightarrow> fetch A a b = (Nop, 0)"
+apply(auto simp: fetch.simps del: nth_of.simps split: block.splits)
+apply(case_tac [!] a, auto simp: t_correct.simps iseven_def)
+done
+
+lemma  [elim]: "\<lbrakk>t_correct A; \<not> isS0 (tstep (a, b, c) A)\<rbrakk> \<Longrightarrow> a \<le> length A div 2"
+apply(rule_tac classical, auto simp: tstep.simps new_tape.simps isS0_def)
+done
+
+lemma [elim]: "\<lbrakk>t_correct A; \<not> isS0 (tstep (a, b, c) A)\<rbrakk> \<Longrightarrow> 0 < a"
+apply(rule_tac classical, simp add: tstep_0 isS0_def)
+done
+
+
+lemma t_merge_pre_eq_step: "\<lbrakk>tstep (a, b, c) A = cf; t_correct A; \<not> isS0 cf\<rbrakk> 
+        \<Longrightarrow> tstep (a, b, c) (A |+| B) = cf"
+apply(subgoal_tac "a \<le> length A div 2 \<and> a \<noteq> 0")
+apply(simp add: tstep.simps)
+apply(case_tac "fetch A a (case c of [] \<Rightarrow> Bk | x # xs \<Rightarrow> x)", simp)
+apply(drule_tac B = B in t_merge_fetch_pre, simp, simp, simp, simp add: isS0_def, auto)
+done
+
+lemma t_merge_pre_eq:  "\<lbrakk>steps (Suc 0, tp) A stp = cf; \<not> isS0 cf; t_correct A\<rbrakk>
+    \<Longrightarrow> steps (Suc 0, tp) (A |+| B) stp = cf"
+proof(induct stp arbitrary: cf)
+  case 0 thus "?case" by(simp add: steps.simps)
+next
+  fix stp cf
+  assume ind: "\<And>cf. \<lbrakk>steps (Suc 0, tp) A stp = cf; \<not> isS0 cf; t_correct A\<rbrakk> 
+                 \<Longrightarrow> steps (Suc 0, tp) (A |+| B) stp = cf"
+  and h1: "steps (Suc 0, tp) A (Suc stp) = cf"
+  and h2: "\<not> isS0 cf"
+  and h3: "t_correct A"
+  from h1 h2 h3 show "steps (Suc 0, tp) (A |+| B) (Suc stp) = cf"
+  proof(simp add: tstep_red, cases "steps (Suc 0, tp) (A) stp", simp)
+    fix a b c
+    assume h4: "tstep (a, b, c) A = cf"
+    and h5: "steps (Suc 0, tp) A stp = (a, b, c)"
+    have "steps (Suc 0, tp) (A |+| B) stp = (a, b, c)"
+    proof(cases a)
+      case 0 thus "?thesis"
+        using h4 h2
+        apply(simp add: tstep_0, cases cf, simp add: isS0_def)
+        done
+    next
+      case (Suc n) thus "?thesis"
+        using h5 h3
+        apply(rule_tac ind, auto simp: isS0_def)
+        done
+    qed
+    thus "tstep (steps (Suc 0, tp) (A |+| B) stp) (A |+| B) = cf"
+      using h4 h5 h3 h2
+      apply(simp)
+      apply(rule t_merge_pre_eq_step, auto)
+      done
+  qed
+qed
+
+declare nth.simps[simp del] tshift.simps[simp del] change_termi_state.simps[simp del]
+
+lemma [simp]: "length (change_termi_state A) = length A"
+by(simp add: change_termi_state.simps)
+
+lemma first_halt_point: "steps (Suc 0, tp) A stp = (0, tp')
+ \<Longrightarrow> \<exists>stp. \<not> isS0 (steps (Suc 0, tp) A stp) \<and> steps (Suc 0, tp) A (Suc stp) = (0, tp')"
+proof(induct stp)
+  case 0  thus "?case" by(simp add: steps.simps)
+next
+  case (Suc n) 
+  fix stp
+  assume ind: "steps (Suc 0, tp) A stp = (0, tp') \<Longrightarrow> 
+       \<exists>stp. \<not> isS0 (steps (Suc 0, tp) A stp) \<and> steps (Suc 0, tp) A (Suc stp) = (0, tp')"
+    and h: "steps (Suc 0, tp) A (Suc stp) = (0, tp')"
+  from h show "\<exists>stp. \<not> isS0 (steps (Suc 0, tp) A stp) \<and> steps (Suc 0, tp) A (Suc stp) = (0, tp')"
+  proof(simp add: tstep_red, cases "steps (Suc 0, tp) A stp", simp, case_tac a)
+    fix a b c
+    assume g1: "a = (0::nat)"
+    and g2: "tstep (a, b, c) A = (0, tp')"
+    and g3: "steps (Suc 0, tp) A stp = (a, b, c)"
+    have "steps (Suc 0, tp) A stp = (0, tp')"
+      using g2 g1 g3
+      by(simp add: tstep_0)
+    hence "\<exists> stp. \<not> isS0 (steps (Suc 0, tp) A stp) \<and> steps (Suc 0, tp) A (Suc stp) = (0, tp')"
+      by(rule ind)
+    thus "\<exists>stp. \<not> isS0 (steps (Suc 0, tp) A stp) \<and> tstep (steps (Suc 0, tp) A stp) A = (0, tp')" 
+      apply(simp add: tstep_red)
+      done
+  next
+    fix a b c nat
+    assume g1: "steps (Suc 0, tp) A stp = (a, b, c)"
+    and g2: "steps (Suc 0, tp) A (Suc stp) = (0, tp')" "a= Suc nat"
+    thus "\<exists>stp. \<not> isS0 (steps (Suc 0, tp) A stp) \<and> tstep (steps (Suc 0, tp) A stp) A = (0, tp')"
+      apply(rule_tac x = stp in exI)
+      apply(simp add: isS0_def tstep_red)
+      done
+  qed
+qed 
+   
+lemma t_merge_pre_halt_same': 
+  "\<lbrakk>\<not> isS0 (steps (Suc 0, tp) A stp) ; steps (Suc 0, tp) A (Suc stp) = (0, tp'); t_correct A\<rbrakk>
+  \<Longrightarrow> steps (Suc 0, tp) (A |+| B) (Suc stp) = (Suc (length A div 2), tp')"    
+proof(simp add: tstep_red, cases "steps (Suc 0, tp) A stp", simp)
+  fix a b c 
+  assume h1: "\<not> isS0 (a, b, c)"
+  and h2: "tstep (a, b, c) A = (0, tp')"
+  and h3: "t_correct A"
+  and h4: "steps (Suc 0, tp) A stp = (a, b, c)"
+  have "steps (Suc 0, tp) (A |+| B) stp = (a, b, c)"
+    using h1 h4 h3
+    apply(rule_tac  t_merge_pre_eq, auto)
+    done
+  moreover have "tstep (a, b, c) (A |+| B) = (Suc (length A div 2), tp')"
+    using h2 h3 h1 h4 
+    apply(simp add: tstep.simps)
+    apply(case_tac " fetch A a (case c of [] \<Rightarrow> Bk | x # xs \<Rightarrow> x)", simp)
+    apply(drule_tac B = B in t_merge_fetch_pre, auto simp: isS0_def intro: s_keep)
+    done
+  ultimately show "tstep (steps (Suc 0, tp) (A |+| B) stp) (A |+| B) = (Suc (length A div 2), tp')"
+    by(simp)
+qed
+
+text {*
+  When Turing machine @{text "A"} and @{text "B"} are combined and the execution
+  of @{text "A"} can termination within @{text "stp"} steps, 
+  the combined machine @{text "A |+| B"} will eventually get into the starting 
+  state of machine @{text "B"}.
+*}
+lemma t_merge_pre_halt_same: "
+  \<lbrakk>steps (Suc 0, tp) A stp = (0, tp'); t_correct A; t_correct B\<rbrakk>
+     \<Longrightarrow> \<exists> stp. steps (Suc 0, tp) (A |+| B) stp = (Suc (length A div 2), tp')"
+proof -
+  assume a_wf: "t_correct A"
+  and b_wf: "t_correct B"
+  and a_ht: "steps (Suc 0, tp) A stp = (0, tp')"
+  have halt_point: "\<exists> stp. \<not> isS0 (steps (Suc 0, tp) A stp) \<and> steps (Suc 0, tp) A (Suc stp) = (0, tp')"
+    using a_ht
+    by(erule_tac first_halt_point)
+  then obtain stp' where "\<not> isS0 (steps (Suc 0, tp) A stp') \<and> steps (Suc 0, tp) A (Suc stp') = (0, tp')"..
+  hence "steps (Suc 0, tp) (A |+| B) (Suc stp') = (Suc (length A div 2), tp')"
+    using a_wf
+    apply(rule_tac t_merge_pre_halt_same', auto)
+    done
+  thus "?thesis" ..
+qed
+
+lemma fetch_0: "fetch p 0 b = (Nop, 0)"
+by(simp add: fetch.simps)
+
+lemma [simp]: "length (tshift B x) = length B"
+by(simp add: tshift.simps)
+
+lemma [simp]: "t_correct A \<Longrightarrow> 2 * (length A div 2) = length A"
+apply(simp add: t_correct.simps iseven_def, auto)
+done
+
+lemma t_merge_fetch_snd: 
+  "\<lbrakk>fetch B a b = (ac, ns); t_correct A; t_correct B; a > 0 \<rbrakk>
+  \<Longrightarrow> fetch (A |+| B) (a + length A div 2) b
+  = (ac, if ns = 0 then 0 else ns + length A div 2)"
+apply(auto simp: fetch.simps t_add.simps split: if_splits block.splits)
+apply(case_tac [!] a, simp_all)
+apply(simp_all add: nth_append change_termi_state.simps tshift.simps)
+done
+
+lemma t_merge_snd_eq_step: 
+  "\<lbrakk>tstep (s, l, r) B = (s', l', r'); t_correct A; t_correct B; s > 0\<rbrakk>
+    \<Longrightarrow> tstep (s + length A div 2, l, r) (A |+| B) = 
+       (if s' = 0 then 0 else s' + length A div 2, l' ,r') "
+apply(simp add: tstep.simps)
+apply(cases "fetch B s (case r of [] \<Rightarrow> Bk | x # xs \<Rightarrow> x)")
+apply(auto simp: t_merge_fetch_snd)
+apply(frule_tac [!] t_merge_fetch_snd, auto)
+done 
+
+text {*
+  Relates the executions of TM @{text "B"}, one is when @{text "B"} is executed alone,
+  the other is the execution when @{text "B"} is in the combined TM.
+*}
+lemma t_merge_snd_eq_steps: 
+  "\<lbrakk>t_correct A; t_correct B; steps (s, l, r) B stp = (s', l', r'); s > 0\<rbrakk>
+  \<Longrightarrow> steps (s + length A div 2, l, r) (A |+| B) stp = 
+      (if s' = 0 then 0 else s' + length A div 2, l', r')"
+proof(induct stp arbitrary: s' l' r')
+  case 0 thus "?case" 
+    by(simp add: steps.simps)
+next
+  fix stp s' l' r'
+  assume ind: "\<And>s' l' r'. \<lbrakk>t_correct A; t_correct B; steps (s, l, r) B stp = (s', l', r'); 0 < s\<rbrakk>
+                   \<Longrightarrow> steps (s + length A div 2, l, r) (A |+| B) stp = 
+                          (if s' = 0 then 0 else s' + length A div 2, l', r')"
+  and h1: "steps (s, l, r) B (Suc stp) = (s', l', r')"
+  and h2: "t_correct A"
+  and h3: "t_correct B"
+  and h4: "0 < s"
+  from h1 show "steps (s + length A div 2, l, r) (A |+| B) (Suc stp) 
+            = (if s' = 0 then 0 else s' + length A div 2, l', r')"
+  proof(simp only: tstep_red, cases "steps (s, l, r) B stp")
+    fix a b c 
+    assume h5: "steps (s, l, r) B stp = (a, b, c)" "tstep (steps (s, l, r) B stp) B = (s', l', r')"
+    hence h6: "(steps (s + length A div 2, l, r) (A |+| B) stp) = 
+                ((if a = 0 then 0 else a + length A div 2, b, c))"
+      using h2 h3 h4
+      by(rule_tac ind, auto)
+    thus "tstep (steps (s + length A div 2, l, r) (A |+| B) stp) (A |+| B) = 
+       (if s' = 0 then 0 else s'+ length A div 2, l', r')"
+      using h5
+    proof(auto)
+      assume "tstep (0, b, c) B = (0, l', r')" thus "tstep (0, b, c) (A |+| B) = (0, l', r')"
+        by(simp add: tstep_0)
+    next
+      assume "tstep (0, b, c) B = (s', l', r')" "0 < s'"
+      thus "tstep (0, b, c) (A |+| B) = (s' + length A div 2, l', r')"
+        by(simp add: tstep_0)
+    next
+      assume "tstep (a, b, c) B = (0, l', r')" "0 < a"
+      thus "tstep (a + length A div 2, b, c) (A |+| B) = (0, l', r')"
+        using h2 h3
+        by(drule_tac t_merge_snd_eq_step, auto)
+    next
+      assume "tstep (a, b, c) B = (s', l', r')" "0 < a" "0 < s'"
+      thus "tstep (a + length A div 2, b, c) (A |+| B) = (s' + length A div 2, l', r')"
+        using h2 h3
+        by(drule_tac t_merge_snd_eq_step, auto)
+    qed
+  qed
+qed
+
+lemma t_merge_snd_halt_eq: 
+  "\<lbrakk>steps (Suc 0, tp) B stp = (0, tp'); t_correct A; t_correct B\<rbrakk>
+  \<Longrightarrow> \<exists>stp. steps (Suc (length A div 2), tp) (A |+| B) stp = (0, tp')"
+apply(case_tac tp, cases tp', simp)
+apply(drule_tac  s = "Suc 0" in t_merge_snd_eq_steps, auto)
+done
+
+lemma t_inj: "\<lbrakk>steps (Suc 0, tp) A stpa = (0, tp1); steps (Suc 0, tp) A stpb = (0, tp2)\<rbrakk> 
+      \<Longrightarrow> tp1 = tp2"
+proof -
+  assume h1: "steps (Suc 0, tp) A stpa = (0, tp1)" 
+  and h2: "steps (Suc 0, tp) A stpb = (0, tp2)"
+  thus "?thesis"
+  proof(cases "stpa < stpb")
+    case True thus "?thesis"
+      using h1 h2
+      apply(drule_tac less_imp_Suc_add, auto)
+      apply(simp del: add_Suc_right add_Suc add: add_Suc_right[THEN sym] steps_add steps_0)
+      done
+  next
+    case False thus "?thesis"
+      using h1 h2
+      apply(drule_tac leI)
+      apply(case_tac "stpb = stpa", auto)
+      apply(subgoal_tac "stpb < stpa")
+      apply(drule_tac less_imp_Suc_add, auto)
+      apply(simp del: add_Suc_right add_Suc add: add_Suc_right[THEN sym] steps_add steps_0)
+      done
+  qed
+qed
+
+type_synonym t_assert = "tape \<Rightarrow> bool"
+
+definition t_imply :: "t_assert \<Rightarrow> t_assert \<Rightarrow> bool" ("_ \<turnstile>-> _" [0, 0] 100)
+  where
+  "t_imply a1 a2 = (\<forall> tp. a1 tp \<longrightarrow> a2 tp)"
+
+
+locale turing_merge =
+  fixes A :: "tprog" and B :: "tprog" and P1 :: "t_assert"
+  and P2 :: "t_assert"
+  and P3 :: "t_assert"
+  and P4 :: "t_assert"
+  and Q1:: "t_assert"
+  and Q2 :: "t_assert"
+  assumes 
+  A_wf : "t_correct A"
+  and B_wf : "t_correct B"
+  and A_halt : "P1 tp \<Longrightarrow> \<exists> stp. let (s, tp') = steps (Suc 0, tp) A stp in s = 0 \<and> Q1 tp'"
+  and B_halt : "P2 tp \<Longrightarrow> \<exists> stp. let (s, tp') = steps (Suc 0, tp) B stp in s = 0 \<and> Q2 tp'"
+  and A_uhalt : "P3 tp \<Longrightarrow> \<not> (\<exists> stp. isS0 (steps (Suc 0, tp) A stp))"
+  and B_uhalt: "P4 tp \<Longrightarrow> \<not> (\<exists> stp. isS0 (steps (Suc 0, tp) B stp))"
+begin
+
+
+text {*
+  The following lemma tries to derive the Hoare logic rule for sequentially combined TMs.
+  It deals with the situtation when both @{text "A"} and @{text "B"} are terminated.
+*}
+
+lemma t_merge_halt: 
+  assumes aimpb: "Q1 \<turnstile>-> P2"
+  shows "P1 \<turnstile>->  \<lambda> tp. (\<exists> stp tp'. steps (Suc 0, tp) (A |+| B)  stp = (0, tp') \<and> Q2 tp')"
+proof(simp add: t_imply_def, auto)
+  fix a b
+  assume h: "P1 (a, b)"
+  hence "\<exists> stp. let (s, tp') = steps (Suc 0, a, b) A stp in s = 0 \<and> Q1 tp'"
+    using A_halt by simp
+  from this obtain stp1 where "let (s, tp') = steps (Suc 0, a, b) A stp1 in s = 0 \<and> Q1 tp'" ..
+  thus "\<exists>stp aa ba. steps (Suc 0, a, b) (A |+| B) stp = (0, aa, ba) \<and> Q2 (aa, ba)"
+  proof(case_tac "steps (Suc 0, a, b) A stp1", simp, erule_tac conjE)
+    fix aa ba c
+    assume g1: "Q1 (ba, c)" 
+      and g2: "steps (Suc 0, a, b) A stp1 = (0, ba, c)"
+    hence "P2 (ba, c)"
+      using aimpb apply(simp add: t_imply_def)
+      done
+    hence "\<exists> stp. let (s, tp') = steps (Suc 0, ba, c) B stp in s = 0 \<and> Q2 tp'"
+      using B_halt by simp
+    from this obtain stp2 where "let (s, tp') = steps (Suc 0, ba, c) B stp2 in s = 0 \<and> Q2 tp'" ..
+    thus "?thesis"
+    proof(case_tac "steps (Suc 0, ba, c) B stp2", simp, erule_tac conjE)
+      fix aa bb ca
+      assume g3: " Q2 (bb, ca)" "steps (Suc 0, ba, c) B stp2 = (0, bb, ca)"
+      have "\<exists> stp. steps (Suc 0, a, b) (A |+| B) stp = (Suc (length A div 2), ba , c)"
+        using g2 A_wf B_wf
+        by(rule_tac t_merge_pre_halt_same, auto)
+      moreover have "\<exists> stp. steps (Suc (length A div 2), ba, c) (A |+| B) stp = (0, bb, ca)"
+        using g3 A_wf B_wf
+        apply(rule_tac t_merge_snd_halt_eq, auto)
+        done
+      ultimately show "\<exists>stp aa ba. steps (Suc 0, a, b) (A |+| B) stp = (0, aa, ba) \<and> Q2 (aa, ba)"
+        apply(erule_tac exE, erule_tac exE)
+        apply(rule_tac x = "stp + stpa" in exI, simp add: steps_add)
+        using g3 by simp
+    qed
+  qed
+qed
+
+lemma  t_merge_uhalt_tmp:
+  assumes B_uh: "\<forall>stp. \<not> isS0 (steps (Suc 0, b, c) B stp)"
+  and merge_ah: "steps (Suc 0, tp) (A |+| B) stpa = (Suc (length A div 2), b, c)" 
+  shows "\<forall> stp. \<not> isS0 (steps (Suc 0, tp) (A |+| B) stp)"
+  using B_uh merge_ah
+apply(rule_tac allI)
+apply(case_tac "stp > stpa")
+apply(erule_tac x = "stp - stpa" in allE)
+apply(case_tac "(steps (Suc 0, b, c) B (stp - stpa))", simp)
+proof -
+  fix stp a ba ca 
+  assume h1: "\<not> isS0 (a, ba, ca)" "stpa < stp"
+  and h2: "steps (Suc 0, b, c) B (stp - stpa) = (a, ba, ca)"
+  have "steps (Suc 0 + length A div 2, b, c) (A |+| B) (stp - stpa) = 
+      (if a = 0 then 0 else a + length A div 2, ba, ca)"
+    using A_wf B_wf h2
+    by(rule_tac t_merge_snd_eq_steps, auto)
+  moreover have "a > 0" using h1 by(simp add: isS0_def)
+  moreover have "\<exists> stpb. stp = stpa + stpb" 
+    using h1 by(rule_tac x = "stp - stpa" in exI, simp)
+  ultimately show "\<not> isS0 (steps (Suc 0, tp) (A |+| B) stp)"
+    using merge_ah
+    by(auto simp: steps_add isS0_def)
+next
+  fix stp
+  assume h: "steps (Suc 0, tp) (A |+| B) stpa = (Suc (length A div 2), b, c)" "\<not> stpa < stp"
+  hence "\<exists> stpb. stpa = stp + stpb" apply(rule_tac x = "stpa - stp" in exI, auto) done
+  thus "\<not> isS0 (steps (Suc 0, tp) (A |+| B) stp)"
+    using h
+    apply(auto)
+    apply(cases "steps (Suc 0, tp) (A |+| B) stp", simp add: steps_add isS0_def steps_0)
+    done
+qed
+
+text {*
+  The following lemma deals with the situation when TM @{text "B"} can not terminate.
+  *}
+
+lemma t_merge_uhalt: 
+  assumes aimpb: "Q1 \<turnstile>-> P4"
+  shows "P1 \<turnstile>-> \<lambda> tp. \<not> (\<exists> stp. isS0 (steps (Suc 0, tp) (A |+| B) stp))"
+proof(simp only: t_imply_def, rule_tac allI, rule_tac impI)
+  fix tp 
+  assume init_asst: "P1 tp"
+  show "\<not> (\<exists>stp. isS0 (steps (Suc 0, tp) (A |+| B) stp))"
+  proof -
+    have "\<exists> stp. let (s, tp') = steps (Suc 0, tp) A stp in s = 0 \<and> Q1 tp'"
+      using A_halt[of tp] init_asst
+      by(simp)
+    from this obtain stpx where "let (s, tp') = steps (Suc 0, tp) A stpx in s = 0 \<and> Q1 tp'" ..
+    thus "?thesis"
+    proof(cases "steps (Suc 0, tp) A stpx", simp, erule_tac conjE)
+      fix a b c
+      assume "Q1 (b, c)"
+        and h3: "steps (Suc 0, tp) A stpx = (0, b, c)"
+      hence h2: "P4 (b, c)"  using aimpb
+        by(simp add: t_imply_def)
+      have "\<exists> stp. steps (Suc 0, tp) (A |+| B) stp = (Suc (length A div 2), b, c)"
+        using h3 A_wf B_wf
+        apply(rule_tac stp = stpx in t_merge_pre_halt_same, auto)
+        done
+      from this obtain stpa where h4:"steps (Suc 0, tp) (A |+| B) stpa = (Suc (length A div 2), b, c)" ..
+      have " \<not> (\<exists> stp. isS0 (steps (Suc 0, b, c) B stp))"
+        using B_uhalt [of "(b, c)"] h2 apply simp
+        done
+      from this and h4 show "\<forall>stp. \<not> isS0 (steps (Suc 0, tp) (A |+| B) stp)"
+        by(rule_tac t_merge_uhalt_tmp, auto)
+    qed
+  qed
+qed
+end
+
+end
+




