--- a/src/HOL/Recdef.thy Mon Aug 31 20:32:00 2009 +0200
+++ b/src/HOL/Recdef.thy Mon Aug 31 20:34:44 2009 +0200
@@ -19,6 +19,65 @@
("Tools/recdef.ML")
begin
+inductive
+ wfrec_rel :: "('a * 'a) set => (('a => 'b) => 'a => 'b) => 'a => 'b => bool"
+ for R :: "('a * 'a) set"
+ and F :: "('a => 'b) => 'a => 'b"
+where
+ wfrecI: "ALL z. (z, x) : R --> wfrec_rel R F z (g z) ==>
+ wfrec_rel R F x (F g x)"
+
+constdefs
+ cut :: "('a => 'b) => ('a * 'a)set => 'a => 'a => 'b"
+ "cut f r x == (%y. if (y,x):r then f y else undefined)"
+
+ adm_wf :: "('a * 'a) set => (('a => 'b) => 'a => 'b) => bool"
+ "adm_wf R F == ALL f g x.
+ (ALL z. (z, x) : R --> f z = g z) --> F f x = F g x"
+
+ wfrec :: "('a * 'a) set => (('a => 'b) => 'a => 'b) => 'a => 'b"
+ [code del]: "wfrec R F == %x. THE y. wfrec_rel R (%f x. F (cut f R x) x) x y"
+
+subsection{*Well-Founded Recursion*}
+
+text{*cut*}
+
+lemma cuts_eq: "(cut f r x = cut g r x) = (ALL y. (y,x):r --> f(y)=g(y))"
+by (simp add: expand_fun_eq cut_def)
+
+lemma cut_apply: "(x,a):r ==> (cut f r a)(x) = f(x)"
+by (simp add: cut_def)
+
+text{*Inductive characterization of wfrec combinator; for details see:
+John Harrison, "Inductive definitions: automation and application"*}
+
+lemma wfrec_unique: "[| adm_wf R F; wf R |] ==> EX! y. wfrec_rel R F x y"
+apply (simp add: adm_wf_def)
+apply (erule_tac a=x in wf_induct)
+apply (rule ex1I)
+apply (rule_tac g = "%x. THE y. wfrec_rel R F x y" in wfrec_rel.wfrecI)
+apply (fast dest!: theI')
+apply (erule wfrec_rel.cases, simp)
+apply (erule allE, erule allE, erule allE, erule mp)
+apply (fast intro: the_equality [symmetric])
+done
+
+lemma adm_lemma: "adm_wf R (%f x. F (cut f R x) x)"
+apply (simp add: adm_wf_def)
+apply (intro strip)
+apply (rule cuts_eq [THEN iffD2, THEN subst], assumption)
+apply (rule refl)
+done
+
+lemma wfrec: "wf(r) ==> wfrec r H a = H (cut (wfrec r H) r a) a"
+apply (simp add: wfrec_def)
+apply (rule adm_lemma [THEN wfrec_unique, THEN the1_equality], assumption)
+apply (rule wfrec_rel.wfrecI)
+apply (intro strip)
+apply (erule adm_lemma [THEN wfrec_unique, THEN theI'])
+done
+
+
text{** This form avoids giant explosions in proofs. NOTE USE OF ==*}
lemma def_wfrec: "[| f==wfrec r H; wf(r) |] ==> f(a) = H (cut f r a) a"
apply auto