author berghofe Tue, 24 Jun 2003 10:37:12 +0200 changeset 14063 e61a310cde02 parent 14062 7f0d5cc52615 child 14064 35d36f43ba06
New proof of weak normalization with program extraction.
--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/src/HOL/Lambda/WeakNorm.thy	Tue Jun 24 10:37:12 2003 +0200
@@ -0,0 +1,604 @@
+(*  Title:      HOL/Lambda/WeakNorm.thy
+    ID:         $Id$
+    Author:     Stefan Berghofer
+*)
+
+header {* Weak normalization for simply-typed lambda calculus *}
+
+theory WeakNorm = Type:
+
+text {*
+Formalization by Stefan Berghofer. Partly based on a paper proof by
+Felix Joachimski and Ralph Matthes \cite{Matthes-Joachimski-AML}.
+*}
+
+
+subsection {* Terms in normal form *}
+
+constdefs
+  listall :: "('a \<Rightarrow> bool) \<Rightarrow> 'a list \<Rightarrow> bool"
+  "listall P xs \<equiv> (\<forall>i. i < length xs \<longrightarrow> P (xs ! i))"
+
+declare listall_def [extraction_expand]
+
+theorem listall_nil: "listall P []"
+
+theorem listall_nil_eq [simp]: "listall P [] = True"
+  by (rules intro: listall_nil)
+
+theorem listall_cons: "P x \<Longrightarrow> listall P xs \<Longrightarrow> listall P (x # xs)"
+  apply (rule allI impI)+
+  apply (case_tac i)
+  apply simp+
+  done
+
+theorem listall_cons_eq [simp]: "listall P (x # xs) = (P x \<and> listall P xs)"
+  apply (rule iffI)
+  prefer 2
+  apply (erule conjE)
+  apply (erule listall_cons)
+  apply assumption
+  apply (unfold listall_def)
+  apply (rule conjI)
+  apply (erule_tac x=0 in allE)
+  apply simp
+  apply simp
+  apply (rule allI)
+  apply (erule_tac x="Suc i" in allE)
+  apply simp
+  done
+
+lemma listall_conj1: "listall (\<lambda>x. P x \<and> Q x) xs \<Longrightarrow> listall P xs"
+  by (induct xs) simp+
+
+lemma listall_conj2: "listall (\<lambda>x. P x \<and> Q x) xs \<Longrightarrow> listall Q xs"
+  by (induct xs) simp+
+
+lemma listall_app: "listall P (xs @ ys) = (listall P xs \<and> listall P ys)"
+  apply (induct xs)
+  apply (rule iffI, simp, simp)
+  apply (rule iffI, simp, simp)
+  done
+
+lemma listall_snoc [simp]: "listall P (xs @ [x]) = (listall P xs \<and> P x)"
+  apply (rule iffI)
+  done
+
+lemma listall_cong [cong, extraction_expand]:
+  "xs = ys \<Longrightarrow> listall P xs = listall P ys"
+  -- {* Currently needed for strange technical reasons *}
+  by (unfold listall_def) simp
+
+consts NF :: "dB set"
+inductive NF
+intros
+  App: "listall (\<lambda>t. t \<in> NF) ts \<Longrightarrow> Var x \<degree>\<degree> ts \<in> NF"
+  Abs: "t \<in> NF \<Longrightarrow> Abs t \<in> NF"
+monos listall_def
+
+lemma nat_eq_dec: "\<And>n::nat. m = n \<or> m \<noteq> n"
+  apply (induct m)
+  apply (case_tac n)
+  apply (case_tac  na)
+  apply (simp only: nat.simps, rules?)+
+  done
+
+lemma nat_le_dec: "\<And>n::nat. m < n \<or> \<not> (m < n)"
+  apply (induct m)
+  apply (case_tac n)
+  apply (case_tac  na)
+  apply (simp del: simp_thms, rules?)+
+  done
+
+lemma App_NF_D: assumes NF: "Var n \<degree>\<degree> ts \<in> NF"
+  shows "listall (\<lambda>t. t \<in> NF) ts" using NF
+  by cases simp_all
+
+
+subsection {* Properties of @{text NF} *}
+
+lemma Var_NF: "Var n \<in> NF"
+  apply (subgoal_tac "Var n \<degree>\<degree> [] \<in> NF")
+   apply simp
+  apply (rule NF.App)
+  apply simp
+  done
+
+lemma subst_terms_NF: "listall (\<lambda>t. t \<in> NF) ts \<Longrightarrow>
+  listall (\<lambda>t. \<forall>i j. t[Var i/j] \<in> NF) ts \<Longrightarrow>
+  listall (\<lambda>t. t \<in> NF) (map (\<lambda>t. t[Var i/j]) ts)"
+  by (induct ts) simp+
+
+lemma subst_Var_NF: "t \<in> NF \<Longrightarrow> (\<And>i j. t[Var i/j] \<in> NF)"
+  apply (induct set: NF)
+  apply simp
+  apply (frule listall_conj1)
+  apply (drule listall_conj2)
+  apply (drule_tac i=i and j=j in subst_terms_NF)
+  apply assumption
+  apply (rule_tac m=x and n=j in nat_eq_dec [THEN disjE, standard])
+  apply simp
+  apply (erule NF.App)
+  apply (rule_tac m=j and n=x in nat_le_dec [THEN disjE, standard])
+  apply simp
+  apply (rules intro: NF.App)
+  apply simp
+  apply (rules intro: NF.App)
+  apply simp
+  apply (rules intro: NF.Abs)
+  done
+
+lemma app_Var_NF: "t \<in> NF \<Longrightarrow> \<exists>t'. t \<degree> Var i \<rightarrow>\<^sub>\<beta>\<^sup>* t' \<and> t' \<in> NF"
+  apply (induct set: NF)
+  apply (subst app_last)
+  apply (rule exI)
+  apply (rule conjI)
+  apply (rule rtrancl_refl)
+  apply (rule NF.App)
+  apply (drule listall_conj1)
+  apply (rule Var_NF)
+  apply (rule exI)
+  apply (rule conjI)
+  apply (rule rtrancl_into_rtrancl)
+  apply (rule rtrancl_refl)
+  apply (rule beta)
+  apply (erule subst_Var_NF)
+  done
+
+lemma lift_terms_NF: "listall (\<lambda>t. t \<in> NF) ts \<Longrightarrow>
+  listall (\<lambda>t. \<forall>i. lift t i \<in> NF) ts \<Longrightarrow>
+  listall (\<lambda>t. t \<in> NF) (map (\<lambda>t. lift t i) ts)"
+  by (induct ts) simp+
+
+lemma lift_NF: "t \<in> NF \<Longrightarrow> (\<And>i. lift t i \<in> NF)"
+  apply (induct set: NF)
+  apply (frule listall_conj1)
+  apply (drule listall_conj2)
+  apply (drule_tac i=i in lift_terms_NF)
+  apply assumption
+  apply (rule_tac m=x and n=i in nat_le_dec [THEN disjE, standard])
+  apply simp
+  apply (rule NF.App)
+  apply assumption
+  apply simp
+  apply (rule NF.App)
+  apply assumption
+  apply simp
+  apply (rule NF.Abs)
+  apply simp
+  done
+
+
+subsection {* Main theorems *}
+
+lemma subst_type_NF:
+  "\<And>t e T u i. t \<in> NF \<Longrightarrow> e\<langle>i:U\<rangle> \<turnstile> t : T \<Longrightarrow> u \<in> NF \<Longrightarrow> e \<turnstile> u : U \<Longrightarrow> \<exists>t'. t[u/i] \<rightarrow>\<^sub>\<beta>\<^sup>* t' \<and> t' \<in> NF"
+  (is "PROP ?P U" is "\<And>t e T u i. _ \<Longrightarrow> PROP ?Q t e T u i U")
+proof (induct U)
+  fix T t
+  let ?R = "\<lambda>t. \<forall>e T' u i.
+    e\<langle>i:T\<rangle> \<turnstile> t : T' \<longrightarrow> u \<in> NF \<longrightarrow> e \<turnstile> u : T \<longrightarrow> (\<exists>t'. t[u/i] \<rightarrow>\<^sub>\<beta>\<^sup>* t' \<and> t' \<in> NF)"
+  assume MI1: "\<And>T1 T2. T = T1 \<Rightarrow> T2 \<Longrightarrow> PROP ?P T1"
+  assume MI2: "\<And>T1 T2. T = T1 \<Rightarrow> T2 \<Longrightarrow> PROP ?P T2"
+  assume "t \<in> NF"
+  thus "\<And>e T' u i. PROP ?Q t e T' u i T"
+  proof induct
+    fix e T' u i assume uNF: "u \<in> NF" and uT: "e \<turnstile> u : T"
+    {
+      case (App ts x e_ T'_ u_ i_)
+      assume appT: "e\<langle>i:T\<rangle> \<turnstile> Var x \<degree>\<degree> ts : T'"
+      from nat_eq_dec show "\<exists>t'. (Var x \<degree>\<degree> ts)[u/i] \<rightarrow>\<^sub>\<beta>\<^sup>* t' \<and> t' \<in> NF"
+      proof
+	assume eq: "x = i"
+	show ?thesis
+	proof (cases ts)
+	  case Nil
+	  with eq have "(Var x \<degree>\<degree> [])[u/i] \<rightarrow>\<^sub>\<beta>\<^sup>* u" by simp
+	  with Nil and uNF show ?thesis by simp rules
+	next
+	  case (Cons a as)
+          with appT have "e\<langle>i:T\<rangle> \<turnstile> Var x \<degree>\<degree> (a # as) : T'" by simp
+	  then obtain Us
+	    where varT': "e\<langle>i:T\<rangle> \<turnstile> Var x : Us \<Rrightarrow> T'"
+	    and argsT': "e\<langle>i:T\<rangle> \<tturnstile> a # as : Us"
+	    by (rule var_app_typesE)
+	  from argsT' obtain T'' Ts where Us: "Us = T'' # Ts"
+	    by (cases Us) (rule FalseE, simp+)
+	  from varT' and Us have varT: "e\<langle>i:T\<rangle> \<turnstile> Var x : T'' \<Rightarrow> Ts \<Rrightarrow> T'"
+	    by simp
+          from varT eq have T: "T = T'' \<Rightarrow> Ts \<Rrightarrow> T'" by cases auto
+          with uT have uT': "e \<turnstile> u : T'' \<Rightarrow> Ts \<Rrightarrow> T'" by simp
+	  from argsT' and Us have argsT: "e\<langle>i:T\<rangle> \<tturnstile> as : Ts" by simp
+	  from argsT' and Us have argT: "e\<langle>i:T\<rangle> \<turnstile> a : T''" by simp
+	  from argT uT refl have aT: "e \<turnstile> a[u/i] : T''" by (rule subst_lemma)
+	  have as: "\<And>Us. e\<langle>i:T\<rangle> \<tturnstile> as : Us \<Longrightarrow> listall ?R as \<Longrightarrow>
+	    \<exists>as'. Var 0 \<degree>\<degree> map (\<lambda>t. lift (t[u/i]) 0) as \<rightarrow>\<^sub>\<beta>\<^sup>*
+	        Var 0 \<degree>\<degree> map (\<lambda>t. lift t 0) as' \<and>
+	      Var 0 \<degree>\<degree> map (\<lambda>t. lift t 0) as' \<in> NF"
+	    (is "\<And>Us. _ \<Longrightarrow> _ \<Longrightarrow> \<exists>as'. ?ex Us as as'")
+	  proof (induct as rule: rev_induct)
+	    case (Nil Us)
+	    with Var_NF have "?ex Us [] []" by simp
+	    thus ?case ..
+	  next
+	    case (snoc b bs Us)
+	    have "e\<langle>i:T\<rangle> \<tturnstile> bs  @ [b] : Us" .
+	    then obtain Vs W where Us: "Us = Vs @ [W]"
+	      and bs: "e\<langle>i:T\<rangle> \<tturnstile> bs : Vs" and bT: "e\<langle>i:T\<rangle> \<turnstile> b : W" by (rule types_snocE)
+	    from snoc have "listall ?R bs" by simp
+	    with bs have "\<exists>bs'. ?ex Vs bs bs'" by (rule snoc)
+	    then obtain bs' where
+	      bsred: "Var 0 \<degree>\<degree> map (\<lambda>t. lift (t[u/i]) 0) bs \<rightarrow>\<^sub>\<beta>\<^sup>*
+	        Var 0 \<degree>\<degree> map (\<lambda>t. lift t 0) bs'"
+	      and bsNF: "Var 0 \<degree>\<degree> map (\<lambda>t. lift t 0) bs' \<in> NF" by rules
+	    from snoc have "?R b" by simp
+	    with bT and uNF and uT have "\<exists>b'. b[u/i] \<rightarrow>\<^sub>\<beta>\<^sup>* b' \<and> b' \<in> NF" by rules
+	    then obtain b' where bred: "b[u/i] \<rightarrow>\<^sub>\<beta>\<^sup>* b'" and bNF: "b' \<in> NF" by rules
+	    from bsNF have "listall (\<lambda>t. t \<in> NF) (map (\<lambda>t. lift t 0) bs')"
+	      by (rule App_NF_D)
+	    moreover have "lift b' 0 \<in> NF" by (rule lift_NF)
+	    ultimately have "listall (\<lambda>t. t \<in> NF) (map (\<lambda>t. lift t 0) (bs' @ [b']))"
+	      by simp
+	    hence "Var 0 \<degree>\<degree> map (\<lambda>t. lift t 0) (bs' @ [b']) \<in> NF" by (rule NF.App)
+	    moreover from bred have "lift (b[u/i]) 0 \<rightarrow>\<^sub>\<beta>\<^sup>* lift b' 0"
+	      by (rule lift_preserves_beta')
+	    with bsred have
+	      "(Var 0 \<degree>\<degree> map (\<lambda>t. lift (t[u/i]) 0) bs) \<degree> lift (b[u/i]) 0 \<rightarrow>\<^sub>\<beta>\<^sup>*
+              (Var 0 \<degree>\<degree> map (\<lambda>t. lift t 0) bs') \<degree> lift b' 0" by (rule rtrancl_beta_App)
+	    ultimately have "?ex Us (bs @ [b]) (bs' @ [b'])" by simp
+	    thus ?case ..
+	  qed
+	  from App and Cons have "listall ?R as" by simp (rules dest: listall_conj2)
+	  with argsT have "\<exists>as'. ?ex Ts as as'" by (rule as)
+	  then obtain as' where
+	    asred: "Var 0 \<degree>\<degree> map (\<lambda>t. lift (t[u/i]) 0) as \<rightarrow>\<^sub>\<beta>\<^sup>*
+	      Var 0 \<degree>\<degree> map (\<lambda>t. lift t 0) as'"
+	    and asNF: "Var 0 \<degree>\<degree> map (\<lambda>t. lift t 0) as' \<in> NF" by rules
+	  from App and Cons have "?R a" by simp
+	  with argT and uNF and uT have "\<exists>a'. a[u/i] \<rightarrow>\<^sub>\<beta>\<^sup>* a' \<and> a' \<in> NF"
+	    by rules
+	  then obtain a' where ared: "a[u/i] \<rightarrow>\<^sub>\<beta>\<^sup>* a'" and aNF: "a' \<in> NF" by rules
+	  from uNF have "lift u 0 \<in> NF" by (rule lift_NF)
+	  hence "\<exists>u'. lift u 0 \<degree> Var 0 \<rightarrow>\<^sub>\<beta>\<^sup>* u' \<and> u' \<in> NF" by (rule app_Var_NF)
+	  then obtain u' where ured: "lift u 0 \<degree> Var 0 \<rightarrow>\<^sub>\<beta>\<^sup>* u'" and u'NF: "u' \<in> NF"
+	    by rules
+	  from T and u'NF have "\<exists>ua. u'[a'/0] \<rightarrow>\<^sub>\<beta>\<^sup>* ua \<and> ua \<in> NF"
+	  proof (rule MI1)
+	    have "e\<langle>0:T''\<rangle> \<turnstile> lift u 0 \<degree> Var 0 : Ts \<Rrightarrow> T'"
+	    proof (rule typing.App)
+	      from uT' show "e\<langle>0:T''\<rangle> \<turnstile> lift u 0 : T'' \<Rightarrow> Ts \<Rrightarrow> T'" by (rule lift_type)
+	      show "e\<langle>0:T''\<rangle> \<turnstile> Var 0 : T''" by (rule typing.Var) simp
+	    qed
+	    with ured show "e\<langle>0:T''\<rangle> \<turnstile> u' : Ts \<Rrightarrow> T'" by (rule subject_reduction')
+	    from ared aT show "e \<turnstile> a' : T''" by (rule subject_reduction')
+	  qed
+	  then obtain ua where uared: "u'[a'/0] \<rightarrow>\<^sub>\<beta>\<^sup>* ua" and uaNF: "ua \<in> NF"
+	    by rules
+	  from ared have "(lift u 0 \<degree> Var 0)[a[u/i]/0] \<rightarrow>\<^sub>\<beta>\<^sup>* (lift u 0 \<degree> Var 0)[a'/0]"
+	    by (rule subst_preserves_beta2')
+	  also from ured have "(lift u 0 \<degree> Var 0)[a'/0] \<rightarrow>\<^sub>\<beta>\<^sup>* u'[a'/0]"
+	    by (rule subst_preserves_beta')
+	  also note uared
+	  finally have "(lift u 0 \<degree> Var 0)[a[u/i]/0] \<rightarrow>\<^sub>\<beta>\<^sup>* ua" .
+	  hence uared': "u \<degree> a[u/i] \<rightarrow>\<^sub>\<beta>\<^sup>* ua" by simp
+	  from T have "\<exists>r. (Var 0 \<degree>\<degree> map (\<lambda>t. lift t 0) as')[ua/0] \<rightarrow>\<^sub>\<beta>\<^sup>* r \<and> r \<in> NF"
+	  proof (rule MI2)
+	    have "e\<langle>0:Ts \<Rrightarrow> T'\<rangle> \<turnstile> Var 0 \<degree>\<degree> map (\<lambda>t. lift (t[u/i]) 0) as : T'"
+	    proof (rule list_app_typeI)
+	      show "e\<langle>0:Ts \<Rrightarrow> T'\<rangle> \<turnstile> Var 0 : Ts \<Rrightarrow> T'" by (rule typing.Var) simp
+	      from uT argsT have "e \<tturnstile> map (\<lambda>t. t[u/i]) as : Ts"
+		by (rule substs_lemma)
+	      hence "e\<langle>0:Ts \<Rrightarrow> T'\<rangle> \<tturnstile> map (\<lambda>t. lift t 0) (map (\<lambda>t. t[u/i]) as) : Ts"
+		by (rule lift_types)
+	      thus "e\<langle>0:Ts \<Rrightarrow> T'\<rangle> \<tturnstile> map (\<lambda>t. lift (t[u/i]) 0) as : Ts"
+		by (simp_all add: map_compose [symmetric] o_def)
+	    qed
+	    with asred show "e\<langle>0:Ts \<Rrightarrow> T'\<rangle> \<turnstile> Var 0 \<degree>\<degree> map (\<lambda>t. lift t 0) as' : T'"
+	      by (rule subject_reduction')
+	    from argT uT refl have "e \<turnstile> a[u/i] : T''" by (rule subst_lemma)
+	    with uT' have "e \<turnstile> u \<degree> a[u/i] : Ts \<Rrightarrow> T'" by (rule typing.App)
+	    with uared' show "e \<turnstile> ua : Ts \<Rrightarrow> T'" by (rule subject_reduction')
+	  qed
+	  then obtain r where rred: "(Var 0 \<degree>\<degree> map (\<lambda>t. lift t 0) as')[ua/0] \<rightarrow>\<^sub>\<beta>\<^sup>* r"
+	    and rnf: "r \<in> NF" by rules
+	  from asred have
+	    "(Var 0 \<degree>\<degree> map (\<lambda>t. lift (t[u/i]) 0) as)[u \<degree> a[u/i]/0] \<rightarrow>\<^sub>\<beta>\<^sup>*
+	    (Var 0 \<degree>\<degree> map (\<lambda>t. lift t 0) as')[u \<degree> a[u/i]/0]"
+	    by (rule subst_preserves_beta')
+	  also from uared' have "(Var 0 \<degree>\<degree> map (\<lambda>t. lift t 0) as')[u \<degree> a[u/i]/0] \<rightarrow>\<^sub>\<beta>\<^sup>*
+	    (Var 0 \<degree>\<degree> map (\<lambda>t. lift t 0) as')[ua/0]" by (rule subst_preserves_beta2')
+	  also note rred
+	  finally have "(Var 0 \<degree>\<degree> map (\<lambda>t. lift (t[u/i]) 0) as)[u \<degree> a[u/i]/0] \<rightarrow>\<^sub>\<beta>\<^sup>* r" .
+	  with rnf Cons eq show ?thesis
+	    by (simp add: map_compose [symmetric] o_def) rules
+	qed
+      next
+	assume neq: "x \<noteq> i"
+	show ?thesis
+	proof -
+	  from appT obtain Us
+	      where varT: "e\<langle>i:T\<rangle> \<turnstile> Var x : Us \<Rrightarrow> T'"
+	      and argsT: "e\<langle>i:T\<rangle> \<tturnstile> ts : Us"
+	    by (rule var_app_typesE)
+	  have ts: "\<And>Us. e\<langle>i:T\<rangle> \<tturnstile> ts : Us \<Longrightarrow> listall ?R ts \<Longrightarrow>
+	    \<exists>ts'. \<forall>x'. Var x' \<degree>\<degree> map (\<lambda>t. t[u/i]) ts \<rightarrow>\<^sub>\<beta>\<^sup>* Var x' \<degree>\<degree> ts' \<and>
+	      Var x' \<degree>\<degree> ts' \<in> NF"
+	    (is "\<And>Us. _ \<Longrightarrow> _ \<Longrightarrow> \<exists>ts'. ?ex Us ts ts'")
+	  proof (induct ts rule: rev_induct)
+	    case (Nil Us)
+	    with Var_NF have "?ex Us [] []" by simp
+	    thus ?case ..
+	  next
+	    case (snoc b bs Us)
+	    have "e\<langle>i:T\<rangle> \<tturnstile> bs  @ [b] : Us" .
+	    then obtain Vs W where Us: "Us = Vs @ [W]"
+	      and bs: "e\<langle>i:T\<rangle> \<tturnstile> bs : Vs" and bT: "e\<langle>i:T\<rangle> \<turnstile> b : W" by (rule types_snocE)
+	    from snoc have "listall ?R bs" by simp
+	    with bs have "\<exists>bs'. ?ex Vs bs bs'" by (rule snoc)
+	    then obtain bs' where
+	      bsred: "\<And>x'. Var x' \<degree>\<degree> map (\<lambda>t. t[u/i]) bs \<rightarrow>\<^sub>\<beta>\<^sup>* Var x' \<degree>\<degree> bs'"
+	      and bsNF: "\<And>x'. Var x' \<degree>\<degree> bs' \<in> NF" by rules
+	    from snoc have "?R b" by simp
+	    with bT and uNF and uT have "\<exists>b'. b[u/i] \<rightarrow>\<^sub>\<beta>\<^sup>* b' \<and> b' \<in> NF" by rules
+	    then obtain b' where bred: "b[u/i] \<rightarrow>\<^sub>\<beta>\<^sup>* b'" and bNF: "b' \<in> NF" by rules
+	    from bsred bred have "\<And>x'. (Var x' \<degree>\<degree> map (\<lambda>t. t[u/i]) bs) \<degree> b[u/i] \<rightarrow>\<^sub>\<beta>\<^sup>*
+              (Var x' \<degree>\<degree> bs') \<degree> b'" by (rule rtrancl_beta_App)
+	    moreover from bsNF [of 0] have "listall (\<lambda>t. t \<in> NF) bs'"
+	      by (rule App_NF_D)
+	    with bNF have "listall (\<lambda>t. t \<in> NF) (bs' @ [b'])" by simp
+	    hence "\<And>x'. Var x' \<degree>\<degree> (bs' @ [b']) \<in> NF" by (rule NF.App)
+	    ultimately have "?ex Us (bs @ [b]) (bs' @ [b'])" by simp
+	    thus ?case ..
+	  qed
+	  from App have "listall ?R ts" by (rules dest: listall_conj2)
+	  with argsT have "\<exists>ts'. ?ex Ts ts ts'" by (rule ts)
+	  then obtain ts' where NF: "?ex Ts ts ts'" ..
+	  from nat_le_dec show ?thesis
+	  proof
+	    assume "i < x"
+	    with NF show ?thesis by simp rules
+	  next
+	    assume "\<not> (i < x)"
+	    with NF neq show ?thesis by (simp add: subst_Var) rules
+	  qed
+	qed
+      qed
+    next
+      case (Abs r e_ T'_ u_ i_)
+      assume absT: "e\<langle>i:T\<rangle> \<turnstile> Abs r : T'"
+      then obtain R S where "e\<langle>0:R\<rangle>\<langle>Suc i:T\<rangle>  \<turnstile> r : S" by (rule abs_typeE) simp
+      moreover have "lift u 0 \<in> NF" by (rule lift_NF)
+      moreover have "e\<langle>0:R\<rangle> \<turnstile> lift u 0 : T" by (rule lift_type)
+      ultimately have "\<exists>t'. r[lift u 0/Suc i] \<rightarrow>\<^sub>\<beta>\<^sup>* t' \<and> t' \<in> NF" by (rule Abs)
+      thus "\<exists>t'. Abs r[u/i] \<rightarrow>\<^sub>\<beta>\<^sup>* t' \<and> t' \<in> NF"
+	by simp (rules intro: rtrancl_beta_Abs NF.Abs)
+    }
+  qed
+qed
+
+
+consts -- {* A computationally relevant copy of @{term "e \<turnstile> t : T"} *}
+  rtyping :: "((nat \<Rightarrow> type) \<times> dB \<times> type) set"
+
+syntax
+  "_rtyping" :: "(nat \<Rightarrow> type) \<Rightarrow> dB \<Rightarrow> type \<Rightarrow> bool"    ("_ |-\<^sub>R _ : _" [50, 50, 50] 50)
+syntax (xsymbols)
+  "_rtyping" :: "(nat \<Rightarrow> type) \<Rightarrow> dB \<Rightarrow> type \<Rightarrow> bool"    ("_ \<turnstile>\<^sub>R _ : _" [50, 50, 50] 50)
+translations
+  "e \<turnstile>\<^sub>R t : T" \<rightleftharpoons> "(e, t, T) \<in> rtyping"
+
+inductive rtyping
+  intros
+    Var: "e x = T \<Longrightarrow> e \<turnstile>\<^sub>R Var x : T"
+    Abs: "e\<langle>0:T\<rangle> \<turnstile>\<^sub>R t : U \<Longrightarrow> e \<turnstile>\<^sub>R Abs t : (T \<Rightarrow> U)"
+    App: "e \<turnstile>\<^sub>R s : T \<Rightarrow> U \<Longrightarrow> e \<turnstile>\<^sub>R t : T \<Longrightarrow> e \<turnstile>\<^sub>R (s \<degree> t) : U"
+
+lemma rtyping_imp_typing: "e \<turnstile>\<^sub>R t : T \<Longrightarrow> e \<turnstile> t : T"
+  apply (induct set: rtyping)
+  apply (erule typing.Var)
+  apply (erule typing.Abs)
+  apply (erule typing.App)
+  apply assumption
+  done
+
+
+theorem type_NF: assumes T: "e \<turnstile>\<^sub>R t : T"
+  shows "\<exists>t'. t \<rightarrow>\<^sub>\<beta>\<^sup>* t' \<and> t' \<in> NF" using T
+proof induct
+  case Var
+  show ?case by (rules intro: Var_NF)
+next
+  case Abs
+  thus ?case by (rules intro: rtrancl_beta_Abs NF.Abs)
+next
+  case (App T U e s t)
+  from App obtain s' t' where
+    sred: "s \<rightarrow>\<^sub>\<beta>\<^sup>* s'" and sNF: "s' \<in> NF"
+    and tred: "t \<rightarrow>\<^sub>\<beta>\<^sup>* t'" and tNF: "t' \<in> NF" by rules
+  have "\<exists>u. (Var 0 \<degree> lift t' 0)[s'/0] \<rightarrow>\<^sub>\<beta>\<^sup>* u \<and> u \<in> NF"
+  proof (rule subst_type_NF)
+    have "lift t' 0 \<in> NF" by (rule lift_NF)
+    hence "listall (\<lambda>t. t \<in> NF) [lift t' 0]" by (rule listall_cons) (rule listall_nil)
+    hence "Var 0 \<degree>\<degree> [lift t' 0] \<in> NF" by (rule NF.App)
+    thus "Var 0 \<degree> lift t' 0 \<in> NF" by simp
+    show "e\<langle>0:T \<Rightarrow> U\<rangle> \<turnstile> Var 0 \<degree> lift t' 0 : U"
+    proof (rule typing.App)
+      show "e\<langle>0:T \<Rightarrow> U\<rangle> \<turnstile> Var 0 : T \<Rightarrow> U"
+      	by (rule typing.Var) simp
+      from tred have "e \<turnstile> t' : T"
+      	by (rule subject_reduction') (rule rtyping_imp_typing)
+      thus "e\<langle>0:T \<Rightarrow> U\<rangle> \<turnstile> lift t' 0 : T"
+      	by (rule lift_type)
+    qed
+    from sred show "e \<turnstile> s' : T \<Rightarrow> U"
+      by (rule subject_reduction') (rule rtyping_imp_typing)
+  qed
+  then obtain u where ured: "s' \<degree> t' \<rightarrow>\<^sub>\<beta>\<^sup>* u" and unf: "u \<in> NF" by simp rules
+  from sred tred have "s \<degree> t \<rightarrow>\<^sub>\<beta>\<^sup>* s' \<degree> t'" by (rule rtrancl_beta_App)
+  hence "s \<degree> t \<rightarrow>\<^sub>\<beta>\<^sup>* u" using ured by (rule rtrancl_trans)
+  with unf show ?case by rules
+qed
+
+
+subsection {* Extracting the program *}
+
+declare NF.induct [ind_realizer]
+declare rtrancl.induct [ind_realizer irrelevant]
+declare rtyping.induct [ind_realizer]
+lemmas [extraction_expand] = trans_def conj_assoc listall_cons_eq
+
+extract type_NF
+
+lemma rtranclR_rtrancl_eq: "((a, b) \<in> rtranclR r) = ((a, b) \<in> rtrancl (Collect r))"
+  apply (rule iffI)
+  apply (erule rtranclR.induct)
+  apply (rule rtrancl_refl)
+  apply (erule rtrancl_into_rtrancl)
+  apply (erule CollectI)
+  apply (erule rtrancl.induct)
+  apply (rule rtranclR.rtrancl_refl)
+  apply (erule rtranclR.rtrancl_into_rtrancl)
+  apply (erule CollectD)
+  done
+
+lemma NFR_imp_NF: "(nf, t) \<in> NFR \<Longrightarrow> t \<in> NF"
+  apply (erule NFR.induct)
+  apply (rule NF.intros)
+  apply (erule NF.intros)
+  done
+
+text_raw {*
+\begin{figure}
+\renewcommand{\isastyle}{\scriptsize\it}%
+@{thm [display,eta_contract=false,margin=100] subst_type_NF_def}
+\renewcommand{\isastyle}{\small\it}%
+\caption{Program extracted from @{text subst_type_NF}}
+\label{fig:extr-subst-type-nf}
+\end{figure}
+
+\begin{figure}
+\renewcommand{\isastyle}{\scriptsize\it}%
+@{thm [display,margin=100] subst_Var_NF_def}
+@{thm [display,margin=100] app_Var_NF_def}
+@{thm [display,margin=100] lift_NF_def}
+@{thm [display,eta_contract=false,margin=100] type_NF_def}
+\renewcommand{\isastyle}{\small\it}%
+\caption{Program extracted from lemmas and main theorem}
+\label{fig:extr-type-nf}
+\end{figure}
+*}
+
+text {*
+The program corresponding to the proof of the central lemma, which
+performs substitution and normalization, is shown in Figure
+\ref{fig:extr-subst-type-nf}. The correctness
+theorem corresponding to the program @{text "subst_type_NF"} is
+@{thm [display,margin=100] subst_type_NF_correctness
+  [simplified rtranclR_rtrancl_eq Collect_mem_eq, no_vars]}
+where @{text NFR} is the realizability predicate corresponding to
+the datatype @{text NFT}, which is inductively defined by the rules
+\pagebreak
+@{thm [display,margin=90] NFR.App [of ts nfs x] NFR.Abs [of nf t]}
+
+The programs corresponding to the main theorem @{text "type_NF"}, as
+well as to some lemmas, are shown in Figure \ref{fig:extr-type-nf}.
+The correctness statement for the main function @{text "type_NF"} is
+@{thm [display,margin=100] type_NF_correctness
+  [simplified rtranclR_rtrancl_eq Collect_mem_eq, no_vars]}
+where the realizability predicate @{text "rtypingR"} corresponding to the
+computationally relevant version of the typing judgement is inductively
+defined by the rules
+@{thm [display,margin=100] rtypingR.Var [no_vars]
+  rtypingR.Abs [of ty, no_vars] rtypingR.App [of ty e s T U ty' t]}
+*}
+
+subsection {* Generating executable code *}
+
+consts_code
+  arbitrary :: "'a"       ("(error \"arbitrary\")")
+  arbitrary :: "'a \<Rightarrow> 'b" ("(fn '_ => error \"arbitrary\")")
+
+generate_code
+  test = "type_NF"
+
+text {*
+The following functions convert between Isabelle's built-in {\tt term}
+datatype and the generated {\tt dB} datatype. This allows to
+generate example terms using Isabelle's parser and inspect
+normalized terms using Isabelle's pretty printer.
+*}
+
+ML {*
+fun nat_of_int 0 = id0
+  | nat_of_int n = Suc (nat_of_int (n-1));
+
+fun int_of_nat id0 = 0
+  | int_of_nat (Suc n) = 1 + int_of_nat n;
+
+fun dBtype_of_typ (Type ("fun", [T, U])) =
+      Fun (dBtype_of_typ T, dBtype_of_typ U)
+  | dBtype_of_typ (TFree (s, _)) = (case explode s of
+        ["'", a] => Atom (nat_of_int (ord a - 97))
+      | _ => error "dBtype_of_typ: variable name")
+  | dBtype_of_typ _ = error "dBtype_of_typ: bad type";
+
+fun dB_of_term (Bound i) = dB_Var (nat_of_int i)
+  | dB_of_term (t $u) = dB_App (dB_of_term t, dB_of_term u) + | dB_of_term (Abs (_, _, t)) = dB_Abs (dB_of_term t) + | dB_of_term _ = error "dB_of_term: bad term"; + +fun term_of_dB Ts (Type ("fun", [T, U])) (dB_Abs dBt) = + Abs ("x", T, term_of_dB (T :: Ts) U dBt) + | term_of_dB Ts _ dBt = term_of_dB' Ts dBt +and term_of_dB' Ts (dB_Var n) = Bound (int_of_nat n) + | term_of_dB' Ts (dB_App (dBt, dBu)) = + let val t = term_of_dB' Ts dBt + in case fastype_of1 (Ts, t) of + Type ("fun", [T, U]) => t$ term_of_dB Ts T dBu
+        | _ => error "term_of_dB: function type expected"
+      end
+  | term_of_dB' _ _ = error "term_of_dB: term not in normal form";
+
+fun typing_of_term Ts e (Bound i) =
+      rtypingT_Var (e, nat_of_int i, dBtype_of_typ (nth_elem (i, Ts)))
+  | typing_of_term Ts e (t \$ u) = (case fastype_of1 (Ts, t) of
+        Type ("fun", [T, U]) => rtypingT_App (e, dB_of_term t,
+          dBtype_of_typ T, dBtype_of_typ U, dB_of_term u,
+          typing_of_term Ts e t, typing_of_term Ts e u)
+      | _ => error "typing_of_term: function type expected")
+  | typing_of_term Ts e (Abs (s, T, t)) =
+      let val dBT = dBtype_of_typ T
+      in rtypingT_Abs (e, dBT, dB_of_term t,
+        dBtype_of_typ (fastype_of1 (T :: Ts, t)),
+        typing_of_term (T :: Ts) (shift e id0 dBT) t)
+      end
+  | typing_of_term _ _ _ = error "typing_of_term: bad term";
+
+fun dummyf _ = error "dummy";
+*}
+
+text {*
+We now try out the extracted program @{text "type_NF"} on some example terms.
+*}
+
+ML {*
+val sg = sign_of (the_context());
+fun rd s = read_cterm sg (s, TypeInfer.logicT);
+
+val ct1 = rd "%f. ((%f x. f (f (f x))) ((%f x. f (f (f (f x)))) f))";
+val (dB1, _) = type_NF (typing_of_term [] dummyf (term_of ct1));
+val ct1' = cterm_of sg (term_of_dB [] (#T (rep_cterm ct1)) dB1);
+
+val ct2 = rd
+  "%f x. (%x. f x x) ((%x. f x x) ((%x. f x x) ((%x. f x x) ((%x. f x x) ((%x. f x x) x)))))";
+val (dB2, _) = type_NF (typing_of_term [] dummyf (term_of ct2));
+val ct2' = cterm_of sg (term_of_dB [] (#T (rep_cterm ct2)) dB2);
+*}
+
+end