diff -r 68e28debe995 -r 191769fc68c3 Higman.thy
--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/Higman.thy	Thu Sep 01 23:18:34 2011 +0000
@@ -0,0 +1,314 @@
+(*  Title:      HOL/Proofs/Extraction/Higman.thy
+    Author:     Stefan Berghofer, TU Muenchen
+    Author:     Monika Seisenberger, LMU Muenchen
+*)
+
+header {* Higman's lemma *}
+
+theory Higman
+imports Main "~~/src/HOL/Library/State_Monad" Random
+begin
+
+text {*
+  Formalization by Stefan Berghofer and Monika Seisenberger,
+  based on Coquand and Fridlender \cite{Coquand93}.
+*}
+
+datatype letter = A | B
+
+inductive emb :: "letter list \<Rightarrow> letter list \<Rightarrow> bool"
+where
+   emb0 [Pure.intro]: "emb [] bs"
+ | emb1 [Pure.intro]: "emb as bs \<Longrightarrow> emb as (b # bs)"
+ | emb2 [Pure.intro]: "emb as bs \<Longrightarrow> emb (a # as) (a # bs)"
+
+inductive L :: "letter list \<Rightarrow> letter list list \<Rightarrow> bool"
+  for v :: "letter list"
+where
+   L0 [Pure.intro]: "emb w v \<Longrightarrow> L v (w # ws)"
+ | L1 [Pure.intro]: "L v ws \<Longrightarrow> L v (w # ws)"
+
+inductive good :: "letter list list \<Rightarrow> bool"
+where
+    good0 [Pure.intro]: "L w ws \<Longrightarrow> good (w # ws)"
+  | good1 [Pure.intro]: "good ws \<Longrightarrow> good (w # ws)"
+
+inductive R :: "letter \<Rightarrow> letter list list \<Rightarrow> letter list list \<Rightarrow> bool"
+  for a :: letter
+where
+    R0 [Pure.intro]: "R a [] []"
+  | R1 [Pure.intro]: "R a vs ws \<Longrightarrow> R a (w # vs) ((a # w) # ws)"
+
+inductive T :: "letter \<Rightarrow> letter list list \<Rightarrow> letter list list \<Rightarrow> bool"
+  for a :: letter
+where
+    T0 [Pure.intro]: "a \<noteq> b \<Longrightarrow> R b ws zs \<Longrightarrow> T a (w # zs) ((a # w) # zs)"
+  | T1 [Pure.intro]: "T a ws zs \<Longrightarrow> T a (w # ws) ((a # w) # zs)"
+  | T2 [Pure.intro]: "a \<noteq> b \<Longrightarrow> T a ws zs \<Longrightarrow> T a ws ((b # w) # zs)"
+
+inductive bar :: "letter list list \<Rightarrow> bool"
+where
+    bar1 [Pure.intro]: "good ws \<Longrightarrow> bar ws"
+  | bar2 [Pure.intro]: "(\<And>w. bar (w # ws)) \<Longrightarrow> bar ws"
+
+theorem prop1: "bar ([] # ws)" by iprover
+
+theorem lemma1: "L as ws \<Longrightarrow> L (a # as) ws"
+  by (erule L.induct, iprover+)
+
+lemma lemma2': "R a vs ws \<Longrightarrow> L as vs \<Longrightarrow> L (a # as) ws"
+  apply (induct set: R)
+  apply (erule L.cases)
+  apply simp+
+  apply (erule L.cases)
+  apply simp_all
+  apply (rule L0)
+  apply (erule emb2)
+  apply (erule L1)
+  done
+
+lemma lemma2: "R a vs ws \<Longrightarrow> good vs \<Longrightarrow> good ws"
+  apply (induct set: R)
+  apply iprover
+  apply (erule good.cases)
+  apply simp_all
+  apply (rule good0)
+  apply (erule lemma2')
+  apply assumption
+  apply (erule good1)
+  done
+
+lemma lemma3': "T a vs ws \<Longrightarrow> L as vs \<Longrightarrow> L (a # as) ws"
+  apply (induct set: T)
+  apply (erule L.cases)
+  apply simp_all
+  apply (rule L0)
+  apply (erule emb2)
+  apply (rule L1)
+  apply (erule lemma1)
+  apply (erule L.cases)
+  apply simp_all
+  apply iprover+
+  done
+
+lemma lemma3: "T a ws zs \<Longrightarrow> good ws \<Longrightarrow> good zs"
+  apply (induct set: T)
+  apply (erule good.cases)
+  apply simp_all
+  apply (rule good0)
+  apply (erule lemma1)
+  apply (erule good1)
+  apply (erule good.cases)
+  apply simp_all
+  apply (rule good0)
+  apply (erule lemma3')
+  apply iprover+
+  done
+
+lemma lemma4: "R a ws zs \<Longrightarrow> ws \<noteq> [] \<Longrightarrow> T a ws zs"
+  apply (induct set: R)
+  apply iprover
+  apply (case_tac vs)
+  apply (erule R.cases)
+  apply simp
+  apply (case_tac a)
+  apply (rule_tac b=B in T0)
+  apply simp
+  apply (rule R0)
+  apply (rule_tac b=A in T0)
+  apply simp
+  apply (rule R0)
+  apply simp
+  apply (rule T1)
+  apply simp
+  done
+
+lemma letter_neq: "(a::letter) \<noteq> b \<Longrightarrow> c \<noteq> a \<Longrightarrow> c = b"
+  apply (case_tac a)
+  apply (case_tac b)
+  apply (case_tac c, simp, simp)
+  apply (case_tac c, simp, simp)
+  apply (case_tac b)
+  apply (case_tac c, simp, simp)
+  apply (case_tac c, simp, simp)
+  done
+
+lemma letter_eq_dec: "(a::letter) = b \<or> a \<noteq> b"
+  apply (case_tac a)
+  apply (case_tac b)
+  apply simp
+  apply simp
+  apply (case_tac b)
+  apply simp
+  apply simp
+  done
+
+theorem prop2:
+  assumes ab: "a \<noteq> b" and bar: "bar xs"
+  shows "\<And>ys zs. bar ys \<Longrightarrow> T a xs zs \<Longrightarrow> T b ys zs \<Longrightarrow> bar zs" using bar
+proof induct
+  fix xs zs assume "T a xs zs" and "good xs"
+  hence "good zs" by (rule lemma3)
+  then show "bar zs" by (rule bar1)
+next
+  fix xs ys
+  assume I: "\<And>w ys zs. bar ys \<Longrightarrow> T a (w # xs) zs \<Longrightarrow> T b ys zs \<Longrightarrow> bar zs"
+  assume "bar ys"
+  thus "\<And>zs. T a xs zs \<Longrightarrow> T b ys zs \<Longrightarrow> bar zs"
+  proof induct
+    fix ys zs assume "T b ys zs" and "good ys"
+    then have "good zs" by (rule lemma3)
+    then show "bar zs" by (rule bar1)
+  next
+    fix ys zs assume I': "\<And>w zs. T a xs zs \<Longrightarrow> T b (w # ys) zs \<Longrightarrow> bar zs"
+    and ys: "\<And>w. bar (w # ys)" and Ta: "T a xs zs" and Tb: "T b ys zs"
+    show "bar zs"
+    proof (rule bar2)
+      fix w
+      show "bar (w # zs)"
+      proof (cases w)
+        case Nil
+        thus ?thesis by simp (rule prop1)
+      next
+        case (Cons c cs)
+        from letter_eq_dec show ?thesis
+        proof
+          assume ca: "c = a"
+          from ab have "bar ((a # cs) # zs)" by (iprover intro: I ys Ta Tb)
+          thus ?thesis by (simp add: Cons ca)
+        next
+          assume "c \<noteq> a"
+          with ab have cb: "c = b" by (rule letter_neq)
+          from ab have "bar ((b # cs) # zs)" by (iprover intro: I' Ta Tb)
+          thus ?thesis by (simp add: Cons cb)
+        qed
+      qed
+    qed
+  qed
+qed
+
+theorem prop3:
+  assumes bar: "bar xs"
+  shows "\<And>zs. xs \<noteq> [] \<Longrightarrow> R a xs zs \<Longrightarrow> bar zs" using bar
+proof induct
+  fix xs zs
+  assume "R a xs zs" and "good xs"
+  then have "good zs" by (rule lemma2)
+  then show "bar zs" by (rule bar1)
+next
+  fix xs zs
+  assume I: "\<And>w zs. w # xs \<noteq> [] \<Longrightarrow> R a (w # xs) zs \<Longrightarrow> bar zs"
+  and xsb: "\<And>w. bar (w # xs)" and xsn: "xs \<noteq> []" and R: "R a xs zs"
+  show "bar zs"
+  proof (rule bar2)
+    fix w
+    show "bar (w # zs)"
+    proof (induct w)
+      case Nil
+      show ?case by (rule prop1)
+    next
+      case (Cons c cs)
+      from letter_eq_dec show ?case
+      proof
+        assume "c = a"
+        thus ?thesis by (iprover intro: I [simplified] R)
+      next
+        from R xsn have T: "T a xs zs" by (rule lemma4)
+        assume "c \<noteq> a"
+        thus ?thesis by (iprover intro: prop2 Cons xsb xsn R T)
+      qed
+    qed
+  qed
+qed
+
+theorem higman: "bar []"
+proof (rule bar2)
+  fix w
+  show "bar [w]"
+  proof (induct w)
+    show "bar [[]]" by (rule prop1)
+  next
+    fix c cs assume "bar [cs]"
+    thus "bar [c # cs]" by (rule prop3) (simp, iprover)
+  qed
+qed
+
+primrec
+  is_prefix :: "'a list \<Rightarrow> (nat \<Rightarrow> 'a) \<Rightarrow> bool"
+where
+    "is_prefix [] f = True"
+  | "is_prefix (x # xs) f = (x = f (length xs) \<and> is_prefix xs f)"
+
+theorem L_idx:
+  assumes L: "L w ws"
+  shows "is_prefix ws f \<Longrightarrow> \<exists>i. emb (f i) w \<and> i < length ws" using L
+proof induct
+  case (L0 v ws)
+  hence "emb (f (length ws)) w" by simp
+  moreover have "length ws < length (v # ws)" by simp
+  ultimately show ?case by iprover
+next
+  case (L1 ws v)
+  then obtain i where emb: "emb (f i) w" and "i < length ws"
+    by simp iprover
+  hence "i < length (v # ws)" by simp
+  with emb show ?case by iprover
+qed
+
+theorem good_idx:
+  assumes good: "good ws"
+  shows "is_prefix ws f \<Longrightarrow> \<exists>i j. emb (f i) (f j) \<and> i < j" using good
+proof induct
+  case (good0 w ws)
+  hence "w = f (length ws)" and "is_prefix ws f" by simp_all
+  with good0 show ?case by (iprover dest: L_idx)
+next
+  case (good1 ws w)
+  thus ?case by simp
+qed
+
+theorem bar_idx:
+  assumes bar: "bar ws"
+  shows "is_prefix ws f \<Longrightarrow> \<exists>i j. emb (f i) (f j) \<and> i < j" using bar
+proof induct
+  case (bar1 ws)
+  thus ?case by (rule good_idx)
+next
+  case (bar2 ws)
+  hence "is_prefix (f (length ws) # ws) f" by simp
+  thus ?case by (rule bar2)
+qed
+
+text {*
+Strong version: yields indices of words that can be embedded into each other.
+*}
+
+theorem higman_idx: "\<exists>(i::nat) j. emb (f i) (f j) \<and> i < j"
+proof (rule bar_idx)
+  show "bar []" by (rule higman)
+  show "is_prefix [] f" by simp
+qed
+
+text {*
+Weak version: only yield sequence containing words
+that can be embedded into each other.
+*}
+
+theorem good_prefix_lemma:
+  assumes bar: "bar ws"
+  shows "is_prefix ws f \<Longrightarrow> \<exists>vs. is_prefix vs f \<and> good vs" using bar
+proof induct
+  case bar1
+  thus ?case by iprover
+next
+  case (bar2 ws)
+  from bar2.prems have "is_prefix (f (length ws) # ws) f" by simp
+  thus ?case by (iprover intro: bar2)
+qed
+
+theorem good_prefix: "\<exists>vs. is_prefix vs f \<and> good vs"
+  using higman
+  by (rule good_prefix_lemma) simp+
+
+
+end