src/HOL/Wellfounded_Recursion.thy
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(*  ID:         $Id$
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    Author:     Tobias Nipkow
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    Copyright   1992  University of Cambridge
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*)
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header {*Well-founded Recursion*}
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theory Wellfounded_Recursion
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imports Transitive_Closure
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begin
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consts
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  wfrec_rel :: "('a * 'a) set => (('a => 'b) => 'a => 'b) => ('a * 'b) set"
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inductive "wfrec_rel R F"
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intros
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  wfrecI: "ALL z. (z, x) : R --> (z, g z) : wfrec_rel R F ==>
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            (x, F g x) : wfrec_rel R F"
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constdefs
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  wf         :: "('a * 'a)set => bool"
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  "wf(r) == (!P. (!x. (!y. (y,x):r --> P(y)) --> P(x)) --> (!x. P(x)))"
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  acyclic :: "('a*'a)set => bool"
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  "acyclic r == !x. (x,x) ~: r^+"
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  cut        :: "('a => 'b) => ('a * 'a)set => 'a => 'a => 'b"
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  "cut f r x == (%y. if (y,x):r then f y else arbitrary)"
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  adm_wf :: "('a * 'a) set => (('a => 'b) => 'a => 'b) => bool"
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  "adm_wf R F == ALL f g x.
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     (ALL z. (z, x) : R --> f z = g z) --> F f x = F g x"
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  wfrec :: "('a * 'a) set => (('a => 'b) => 'a => 'b) => 'a => 'b"
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  "wfrec R F == %x. THE y. (x, y) : wfrec_rel R (%f x. F (cut f R x) x)"
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axclass wellorder \<subseteq> linorder
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  wf: "wf {(x,y::'a::ord). x<y}"
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lemma wfUNIVI: 
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   "(!!P x. (ALL x. (ALL y. (y,x) : r --> P(y)) --> P(x)) ==> P(x)) ==> wf(r)"
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by (unfold wf_def, blast)
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text{*Restriction to domain @{term A} and range @{term B}.  If @{term r} is
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    well-founded over their intersection, then @{term "wf r"}*}
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lemma wfI: 
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 "[| r \<subseteq> A <*> B; 
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     !!x P. [|\<forall>x. (\<forall>y. (y,x) : r --> P y) --> P x;  x : A; x : B |] ==> P x |]
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  ==>  wf r"
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by (unfold wf_def, blast)
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lemma wf_induct: 
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    "[| wf(r);           
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        !!x.[| ALL y. (y,x): r --> P(y) |] ==> P(x)  
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     |]  ==>  P(a)"
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by (unfold wf_def, blast)
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lemmas wf_induct_rule = wf_induct [rule_format, consumes 1, case_names less, induct set: wf]
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lemma wf_not_sym [rule_format]: "wf(r) ==> ALL x. (a,x):r --> (x,a)~:r"
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by (erule_tac a=a in wf_induct, blast)
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(* [| wf r;  ~Z ==> (a,x) : r;  (x,a) ~: r ==> Z |] ==> Z *)
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lemmas wf_asym = wf_not_sym [elim_format]
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lemma wf_not_refl [simp]: "wf(r) ==> (a,a) ~: r"
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by (blast elim: wf_asym)
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(* [| wf r;  (a,a) ~: r ==> PROP W |] ==> PROP W *)
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lemmas wf_irrefl = wf_not_refl [elim_format]
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text{*transitive closure of a well-founded relation is well-founded! *}
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lemma wf_trancl: "wf(r) ==> wf(r^+)"
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apply (subst wf_def, clarify)
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apply (rule allE, assumption)
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  --{*Retains the universal formula for later use!*}
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apply (erule mp)
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apply (erule_tac a = x in wf_induct)
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apply (blast elim: tranclE)
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done
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lemma wf_converse_trancl: "wf (r^-1) ==> wf ((r^+)^-1)"
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apply (subst trancl_converse [symmetric])
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apply (erule wf_trancl)
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done
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subsubsection{*Other simple well-foundedness results*}
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text{*Minimal-element characterization of well-foundedness*}
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lemma wf_eq_minimal: "wf r = (\<forall>Q x. x\<in>Q --> (\<exists>z\<in>Q. \<forall>y. (y,z)\<in>r --> y\<notin>Q))"
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proof (intro iffI strip)
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  fix Q::"'a set" and x
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  assume "wf r" and "x \<in> Q"
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  thus "\<exists>z\<in>Q. \<forall>y. (y, z) \<in> r \<longrightarrow> y \<notin> Q"
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    by (unfold wf_def, 
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        blast dest: spec [of _ "%x. x\<in>Q \<longrightarrow> (\<exists>z\<in>Q. \<forall>y. (y,z) \<in> r \<longrightarrow> y\<notin>Q)"]) 
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next
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  assume "\<forall>Q x. x \<in> Q \<longrightarrow> (\<exists>z\<in>Q. \<forall>y. (y, z) \<in> r \<longrightarrow> y \<notin> Q)"
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  thus "wf r" by (unfold wf_def, force)
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qed
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text{*Well-foundedness of subsets*}
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lemma wf_subset: "[| wf(r);  p<=r |] ==> wf(p)"
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apply (simp (no_asm_use) add: wf_eq_minimal)
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apply fast
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done
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text{*Well-foundedness of the empty relation*}
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lemma wf_empty [iff]: "wf({})"
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by (simp add: wf_def)
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lemma wf_Int1: "wf r ==> wf (r Int r')"
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by (erule wf_subset, rule Int_lower1)
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lemma wf_Int2: "wf r ==> wf (r' Int r)"
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by (erule wf_subset, rule Int_lower2)
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text{*Well-foundedness of insert*}
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lemma wf_insert [iff]: "wf(insert (y,x) r) = (wf(r) & (x,y) ~: r^*)"
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apply (rule iffI)
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 apply (blast elim: wf_trancl [THEN wf_irrefl]
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              intro: rtrancl_into_trancl1 wf_subset 
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                     rtrancl_mono [THEN [2] rev_subsetD])
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apply (simp add: wf_eq_minimal, safe)
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apply (rule allE, assumption, erule impE, blast) 
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apply (erule bexE)
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apply (rename_tac "a", case_tac "a = x")
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 prefer 2
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apply blast 
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apply (case_tac "y:Q")
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 prefer 2 apply blast
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apply (rule_tac x = "{z. z:Q & (z,y) : r^*}" in allE)
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 apply assumption
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apply (erule_tac V = "ALL Q. (EX x. x : Q) --> ?P Q" in thin_rl) 
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  --{*essential for speed*}
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txt{*Blast with new substOccur fails*}
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apply (fast intro: converse_rtrancl_into_rtrancl)
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done
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text{*Well-foundedness of image*}
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lemma wf_prod_fun_image: "[| wf r; inj f |] ==> wf(prod_fun f f ` r)"
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apply (simp only: wf_eq_minimal, clarify)
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apply (case_tac "EX p. f p : Q")
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apply (erule_tac x = "{p. f p : Q}" in allE)
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apply (fast dest: inj_onD, blast)
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done
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subsubsection{*Well-Foundedness Results for Unions*}
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text{*Well-foundedness of indexed union with disjoint domains and ranges*}
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lemma wf_UN: "[| ALL i:I. wf(r i);  
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         ALL i:I. ALL j:I. r i ~= r j --> Domain(r i) Int Range(r j) = {}  
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      |] ==> wf(UN i:I. r i)"
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apply (simp only: wf_eq_minimal, clarify)
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apply (rename_tac A a, case_tac "EX i:I. EX a:A. EX b:A. (b,a) : r i")
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 prefer 2
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 apply force 
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apply clarify
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apply (drule bspec, assumption)  
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apply (erule_tac x="{a. a:A & (EX b:A. (b,a) : r i) }" in allE)
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apply (blast elim!: allE)  
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done
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lemma wf_Union: 
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 "[| ALL r:R. wf r;  
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     ALL r:R. ALL s:R. r ~= s --> Domain r Int Range s = {}  
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  |] ==> wf(Union R)"
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apply (simp add: Union_def)
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apply (blast intro: wf_UN)
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done
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(*Intuition: we find an (R u S)-min element of a nonempty subset A
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             by case distinction.
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  1. There is a step a -R-> b with a,b : A.
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     Pick an R-min element z of the (nonempty) set {a:A | EX b:A. a -R-> b}.
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     By definition, there is z':A s.t. z -R-> z'. Because z is R-min in the
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     subset, z' must be R-min in A. Because z' has an R-predecessor, it cannot
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     have an S-successor and is thus S-min in A as well.
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  2. There is no such step.
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     Pick an S-min element of A. In this case it must be an R-min
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     element of A as well.
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*)
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lemma wf_Un:
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     "[| wf r; wf s; Domain r Int Range s = {} |] ==> wf(r Un s)"
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apply (simp only: wf_eq_minimal, clarify) 
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apply (rename_tac A a)
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apply (case_tac "EX a:A. EX b:A. (b,a) : r") 
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 prefer 2
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 apply simp
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 apply (drule_tac x=A in spec)+
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 apply blast 
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apply (erule_tac x="{a. a:A & (EX b:A. (b,a) : r) }" in allE)+
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apply (blast elim!: allE)  
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done
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subsubsection {*acyclic*}
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lemma acyclicI: "ALL x. (x, x) ~: r^+ ==> acyclic r"
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by (simp add: acyclic_def)
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lemma wf_acyclic: "wf r ==> acyclic r"
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apply (simp add: acyclic_def)
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apply (blast elim: wf_trancl [THEN wf_irrefl])
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done
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lemma acyclic_insert [iff]:
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     "acyclic(insert (y,x) r) = (acyclic r & (x,y) ~: r^*)"
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apply (simp add: acyclic_def trancl_insert)
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apply (blast intro: rtrancl_trans)
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done
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lemma acyclic_converse [iff]: "acyclic(r^-1) = acyclic r"
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by (simp add: acyclic_def trancl_converse)
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lemma acyclic_impl_antisym_rtrancl: "acyclic r ==> antisym(r^*)"
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apply (simp add: acyclic_def antisym_def)
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apply (blast elim: rtranclE intro: rtrancl_into_trancl1 rtrancl_trancl_trancl)
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done
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(* Other direction:
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acyclic = no loops
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antisym = only self loops
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Goalw [acyclic_def,antisym_def] "antisym( r^* ) ==> acyclic(r - Id)
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==> antisym( r^* ) = acyclic(r - Id)";
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*)
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lemma acyclic_subset: "[| acyclic s; r <= s |] ==> acyclic r"
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apply (simp add: acyclic_def)
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apply (blast intro: trancl_mono)
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done
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subsection{*Well-Founded Recursion*}
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text{*cut*}
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lemma cuts_eq: "(cut f r x = cut g r x) = (ALL y. (y,x):r --> f(y)=g(y))"
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by (simp add: expand_fun_eq cut_def)
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lemma cut_apply: "(x,a):r ==> (cut f r a)(x) = f(x)"
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by (simp add: cut_def)
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text{*Inductive characterization of wfrec combinator; for details see:  
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John Harrison, "Inductive definitions: automation and application"*}
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lemma wfrec_unique: "[| adm_wf R F; wf R |] ==> EX! y. (x, y) : wfrec_rel R F"
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apply (simp add: adm_wf_def)
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apply (erule_tac a=x in wf_induct) 
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apply (rule ex1I)
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apply (rule_tac g = "%x. THE y. (x, y) : wfrec_rel R F" in wfrec_rel.wfrecI)
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apply (fast dest!: theI')
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apply (erule wfrec_rel.cases, simp)
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apply (erule allE, erule allE, erule allE, erule mp)
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apply (fast intro: the_equality [symmetric])
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done
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lemma adm_lemma: "adm_wf R (%f x. F (cut f R x) x)"
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apply (simp add: adm_wf_def)
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apply (intro strip)
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apply (rule cuts_eq [THEN iffD2, THEN subst], assumption)
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apply (rule refl)
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done
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lemma wfrec: "wf(r) ==> wfrec r H a = H (cut (wfrec r H) r a) a"
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apply (simp add: wfrec_def)
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apply (rule adm_lemma [THEN wfrec_unique, THEN the1_equality], assumption)
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apply (rule wfrec_rel.wfrecI)
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apply (intro strip)
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apply (erule adm_lemma [THEN wfrec_unique, THEN theI'])
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done
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text{** This form avoids giant explosions in proofs.  NOTE USE OF ==*}
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lemma def_wfrec: "[| f==wfrec r H;  wf(r) |] ==> f(a) = H (cut f r a) a"
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apply auto
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apply (blast intro: wfrec)
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done
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subsection {* Code generator setup *}
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consts_code
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  "wfrec"   ("\<module>wfrec?")
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attach {*
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fun wfrec f x = f (wfrec f) x;
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*}
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setup {*
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  CodegenPackage.add_appconst ("wfrec", CodegenPackage.appgen_wfrec)
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*}
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code_constapp
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  wfrec
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    ml (target_atom "(let fun wfrec f x = f (wfrec f) x in wfrec end)")
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    haskell (target_atom "(wfrec where wfrec f x = f (wfrec f) x)")
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15341
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subsection{*Variants for TFL: the Recdef Package*}
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lemma tfl_wf_induct: "ALL R. wf R -->  
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       (ALL P. (ALL x. (ALL y. (y,x):R --> P y) --> P x) --> (ALL x. P x))"
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apply clarify
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apply (rule_tac r = R and P = P and a = x in wf_induct, assumption, blast)
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done
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lemma tfl_cut_apply: "ALL f R. (x,a):R --> (cut f R a)(x) = f(x)"
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apply clarify
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apply (rule cut_apply, assumption)
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done
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lemma tfl_wfrec:
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     "ALL M R f. (f=wfrec R M) --> wf R --> (ALL x. f x = M (cut f R x) x)"
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apply clarify
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apply (erule wfrec)
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done
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subsection {*LEAST and wellorderings*}
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text{* See also @{text wf_linord_ex_has_least} and its consequences in
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 @{text Wellfounded_Relations.ML}*}
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lemma wellorder_Least_lemma [rule_format]:
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     "P (k::'a::wellorder) --> P (LEAST x. P(x)) & (LEAST x. P(x)) <= k"
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apply (rule_tac a = k in wf [THEN wf_induct])
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apply (rule impI)
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apply (rule classical)
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apply (rule_tac s = x in Least_equality [THEN ssubst], auto)
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apply (auto simp add: linorder_not_less [symmetric])
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done
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lemmas LeastI   = wellorder_Least_lemma [THEN conjunct1, standard]
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lemmas Least_le = wellorder_Least_lemma [THEN conjunct2, standard]
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15950
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-- "The following 3 lemmas are due to Brian Huffman"
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lemma LeastI_ex: "EX x::'a::wellorder. P x ==> P (Least P)"
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apply (erule exE)
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apply (erule LeastI)
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done
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lemma LeastI2:
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  "[| P (a::'a::wellorder); !!x. P x ==> Q x |] ==> Q (Least P)"
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by (blast intro: LeastI)
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lemma LeastI2_ex:
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  "[| EX a::'a::wellorder. P a; !!x. P x ==> Q x |] ==> Q (Least P)"
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by (blast intro: LeastI_ex)
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15341
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lemma not_less_Least: "[| k < (LEAST x. P x) |] ==> ~P (k::'a::wellorder)"
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apply (simp (no_asm_use) add: linorder_not_le [symmetric])
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apply (erule contrapos_nn)
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apply (erule Least_le)
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done
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ML
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{*
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val wf_def = thm "wf_def";
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val wfUNIVI = thm "wfUNIVI";
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val wfI = thm "wfI";
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val wf_induct = thm "wf_induct";
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val wf_not_sym = thm "wf_not_sym";
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val wf_asym = thm "wf_asym";
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val wf_not_refl = thm "wf_not_refl";
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val wf_irrefl = thm "wf_irrefl";
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val wf_trancl = thm "wf_trancl";
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val wf_converse_trancl = thm "wf_converse_trancl";
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val wf_eq_minimal = thm "wf_eq_minimal";
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val wf_subset = thm "wf_subset";
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val wf_empty = thm "wf_empty";
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val wf_insert = thm "wf_insert";
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val wf_UN = thm "wf_UN";
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val wf_Union = thm "wf_Union";
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val wf_Un = thm "wf_Un";
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val wf_prod_fun_image = thm "wf_prod_fun_image";
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val acyclicI = thm "acyclicI";
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val wf_acyclic = thm "wf_acyclic";
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val acyclic_insert = thm "acyclic_insert";
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val acyclic_converse = thm "acyclic_converse";
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val acyclic_impl_antisym_rtrancl = thm "acyclic_impl_antisym_rtrancl";
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val acyclic_subset = thm "acyclic_subset";
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val cuts_eq = thm "cuts_eq";
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val cut_apply = thm "cut_apply";
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val wfrec_unique = thm "wfrec_unique";
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val wfrec = thm "wfrec";
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val def_wfrec = thm "def_wfrec";
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val tfl_wf_induct = thm "tfl_wf_induct";
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val tfl_cut_apply = thm "tfl_cut_apply";
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val tfl_wfrec = thm "tfl_wfrec";
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val LeastI = thm "LeastI";
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val Least_le = thm "Least_le";
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val not_less_Least = thm "not_less_Least";
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*}
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10213
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end