src/ZF/WF.thy
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(*  Title:      ZF/WF.thy
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    Author:     Tobias Nipkow and Lawrence C Paulson
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    Copyright   1994  University of Cambridge
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Derived first for transitive relations, and finally for arbitrary WF relations
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via wf_trancl and trans_trancl.
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It is difficult to derive this general case directly, using r^+ instead of
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r.  In is_recfun, the two occurrences of the relation must have the same
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form.  Inserting r^+ in the_recfun or wftrec yields a recursion rule with
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r^+ -`` {a} instead of r-``{a}.  This recursion rule is stronger in
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principle, but harder to use, especially to prove wfrec_eclose_eq in
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epsilon.ML.  Expanding out the definition of wftrec in wfrec would yield
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a mess.
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*)
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header{*Well-Founded Recursion*}
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theory WF imports Trancl begin
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definition
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  wf           :: "i=>o"  where
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    (*r is a well-founded relation*)
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    "wf(r) == \<forall>Z. Z=0 | (\<exists>x\<in>Z. \<forall>y. <y,x>:r \<longrightarrow> ~ y \<in> Z)"
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definition
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  wf_on        :: "[i,i]=>o"                      ("wf[_]'(_')")  where
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    (*r is well-founded on A*)
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    "wf_on(A,r) == wf(r \<inter> A*A)"
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definition
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  is_recfun    :: "[i, i, [i,i]=>i, i] =>o"  where
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    "is_recfun(r,a,H,f) == (f = (\<lambda>x\<in>r-``{a}. H(x, restrict(f, r-``{x}))))"
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definition
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  the_recfun   :: "[i, i, [i,i]=>i] =>i"  where
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    "the_recfun(r,a,H) == (THE f. is_recfun(r,a,H,f))"
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definition
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  wftrec :: "[i, i, [i,i]=>i] =>i"  where
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    "wftrec(r,a,H) == H(a, the_recfun(r,a,H))"
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definition
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  wfrec :: "[i, i, [i,i]=>i] =>i"  where
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    (*public version.  Does not require r to be transitive*)
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    "wfrec(r,a,H) == wftrec(r^+, a, %x f. H(x, restrict(f,r-``{x})))"
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definition
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  wfrec_on     :: "[i, i, i, [i,i]=>i] =>i"       ("wfrec[_]'(_,_,_')")  where
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    "wfrec[A](r,a,H) == wfrec(r \<inter> A*A, a, H)"
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subsection{*Well-Founded Relations*}
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subsubsection{*Equivalences between @{term wf} and @{term wf_on}*}
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lemma wf_imp_wf_on: "wf(r) ==> wf[A](r)"
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by (unfold wf_def wf_on_def, force)
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lemma wf_on_imp_wf: "[|wf[A](r); r \<subseteq> A*A|] ==> wf(r)"
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by (simp add: wf_on_def subset_Int_iff)
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lemma wf_on_field_imp_wf: "wf[field(r)](r) ==> wf(r)"
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by (unfold wf_def wf_on_def, fast)
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lemma wf_iff_wf_on_field: "wf(r) \<longleftrightarrow> wf[field(r)](r)"
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by (blast intro: wf_imp_wf_on wf_on_field_imp_wf)
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lemma wf_on_subset_A: "[| wf[A](r);  B<=A |] ==> wf[B](r)"
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by (unfold wf_on_def wf_def, fast)
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lemma wf_on_subset_r: "[| wf[A](r); s<=r |] ==> wf[A](s)"
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by (unfold wf_on_def wf_def, fast)
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lemma wf_subset: "[|wf(s); r<=s|] ==> wf(r)"
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by (simp add: wf_def, fast)
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subsubsection{*Introduction Rules for @{term wf_on}*}
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text{*If every non-empty subset of @{term A} has an @{term r}-minimal element
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   then we have @{term "wf[A](r)"}.*}
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lemma wf_onI:
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 assumes prem: "!!Z u. [| Z<=A;  u \<in> Z;  \<forall>x\<in>Z. \<exists>y\<in>Z. <y,x>:r |] ==> False"
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 shows         "wf[A](r)"
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apply (unfold wf_on_def wf_def)
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apply (rule equals0I [THEN disjCI, THEN allI])
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apply (rule_tac Z = Z in prem, blast+)
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done
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text{*If @{term r} allows well-founded induction over @{term A}
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   then we have @{term "wf[A](r)"}.   Premise is equivalent to
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  @{prop "!!B. \<forall>x\<in>A. (\<forall>y. <y,x>: r \<longrightarrow> y \<in> B) \<longrightarrow> x \<in> B ==> A<=B"} *}
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lemma wf_onI2:
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 assumes prem: "!!y B. [| \<forall>x\<in>A. (\<forall>y\<in>A. <y,x>:r \<longrightarrow> y \<in> B) \<longrightarrow> x \<in> B;   y \<in> A |]
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                       ==> y \<in> B"
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 shows         "wf[A](r)"
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apply (rule wf_onI)
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apply (rule_tac c=u in prem [THEN DiffE])
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  prefer 3 apply blast
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 apply fast+
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done
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subsubsection{*Well-founded Induction*}
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text{*Consider the least @{term z} in @{term "domain(r)"} such that
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  @{term "P(z)"} does not hold...*}
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lemma wf_induct_raw:
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    "[| wf(r);
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        !!x.[| \<forall>y. <y,x>: r \<longrightarrow> P(y) |] ==> P(x) |]
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     ==> P(a)"
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apply (unfold wf_def)
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apply (erule_tac x = "{z \<in> domain(r). ~ P(z)}" in allE)
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apply blast
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done
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lemmas wf_induct = wf_induct_raw [rule_format, consumes 1, case_names step, induct set: wf]
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text{*The form of this rule is designed to match @{text wfI}*}
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lemma wf_induct2:
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    "[| wf(r);  a \<in> A;  field(r)<=A;
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        !!x.[| x \<in> A;  \<forall>y. <y,x>: r \<longrightarrow> P(y) |] ==> P(x) |]
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     ==>  P(a)"
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apply (erule_tac P="a \<in> A" in rev_mp)
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apply (erule_tac a=a in wf_induct, blast)
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done
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lemma field_Int_square: "field(r \<inter> A*A) \<subseteq> A"
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by blast
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lemma wf_on_induct_raw [consumes 2, induct set: wf_on]:
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    "[| wf[A](r);  a \<in> A;
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        !!x.[| x \<in> A;  \<forall>y\<in>A. <y,x>: r \<longrightarrow> P(y) |] ==> P(x)
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     |]  ==>  P(a)"
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apply (unfold wf_on_def)
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apply (erule wf_induct2, assumption)
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apply (rule field_Int_square, blast)
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done
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lemmas wf_on_induct =
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  wf_on_induct_raw [rule_format, consumes 2, case_names step, induct set: wf_on]
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text{*If @{term r} allows well-founded induction
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   then we have @{term "wf(r)"}.*}
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lemma wfI:
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    "[| field(r)<=A;
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        !!y B. [| \<forall>x\<in>A. (\<forall>y\<in>A. <y,x>:r \<longrightarrow> y \<in> B) \<longrightarrow> x \<in> B;  y \<in> A|]
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               ==> y \<in> B |]
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     ==>  wf(r)"
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apply (rule wf_on_subset_A [THEN wf_on_field_imp_wf])
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apply (rule wf_onI2)
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 prefer 2 apply blast
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apply blast
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done
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subsection{*Basic Properties of Well-Founded Relations*}
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lemma wf_not_refl: "wf(r) ==> <a,a> \<notin> r"
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by (erule_tac a=a in wf_induct, blast)
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lemma wf_not_sym [rule_format]: "wf(r) ==> \<forall>x. <a,x>:r \<longrightarrow> <x,a> \<notin> r"
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by (erule_tac a=a in wf_induct, blast)
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(* @{term"[| wf(r);  <a,x> \<in> r;  ~P ==> <x,a> \<in> r |] ==> P"} *)
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lemmas wf_asym = wf_not_sym [THEN swap]
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lemma wf_on_not_refl: "[| wf[A](r); a \<in> A |] ==> <a,a> \<notin> r"
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by (erule_tac a=a in wf_on_induct, assumption, blast)
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lemma wf_on_not_sym [rule_format]:
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     "[| wf[A](r);  a \<in> A |] ==> \<forall>b\<in>A. <a,b>:r \<longrightarrow> <b,a>\<notin>r"
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apply (erule_tac a=a in wf_on_induct, assumption, blast)
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done
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lemma wf_on_asym:
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     "[| wf[A](r);  ~Z ==> <a,b> \<in> r;
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         <b,a> \<notin> r ==> Z; ~Z ==> a \<in> A; ~Z ==> b \<in> A |] ==> Z"
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by (blast dest: wf_on_not_sym)
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(*Needed to prove well_ordI.  Could also reason that wf[A](r) means
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  wf(r \<inter> A*A);  thus wf( (r \<inter> A*A)^+ ) and use wf_not_refl *)
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lemma wf_on_chain3:
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     "[| wf[A](r); <a,b>:r; <b,c>:r; <c,a>:r; a \<in> A; b \<in> A; c \<in> A |] ==> P"
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apply (subgoal_tac "\<forall>y\<in>A. \<forall>z\<in>A. <a,y>:r \<longrightarrow> <y,z>:r \<longrightarrow> <z,a>:r \<longrightarrow> P",
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       blast)
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apply (erule_tac a=a in wf_on_induct, assumption, blast)
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done
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text{*transitive closure of a WF relation is WF provided
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  @{term A} is downward closed*}
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lemma wf_on_trancl:
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    "[| wf[A](r);  r-``A \<subseteq> A |] ==> wf[A](r^+)"
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apply (rule wf_onI2)
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apply (frule bspec [THEN mp], assumption+)
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apply (erule_tac a = y in wf_on_induct, assumption)
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apply (blast elim: tranclE, blast)
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done
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lemma wf_trancl: "wf(r) ==> wf(r^+)"
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apply (simp add: wf_iff_wf_on_field)
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apply (rule wf_on_subset_A)
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 apply (erule wf_on_trancl)
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 apply blast
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apply (rule trancl_type [THEN field_rel_subset])
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done
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text{*@{term "r-``{a}"} is the set of everything under @{term a} in @{term r}*}
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lemmas underI = vimage_singleton_iff [THEN iffD2]
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lemmas underD = vimage_singleton_iff [THEN iffD1]
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subsection{*The Predicate @{term is_recfun}*}
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lemma is_recfun_type: "is_recfun(r,a,H,f) ==> f \<in> r-``{a} -> range(f)"
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apply (unfold is_recfun_def)
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apply (erule ssubst)
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apply (rule lamI [THEN rangeI, THEN lam_type], assumption)
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done
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lemmas is_recfun_imp_function = is_recfun_type [THEN fun_is_function]
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lemma apply_recfun:
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    "[| is_recfun(r,a,H,f); <x,a>:r |] ==> f`x = H(x, restrict(f,r-``{x}))"
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apply (unfold is_recfun_def)
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  txt{*replace f only on the left-hand side*}
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apply (erule_tac P = "%x.?t(x) = ?u" in ssubst)
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apply (simp add: underI)
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done
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lemma is_recfun_equal [rule_format]:
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     "[| wf(r);  trans(r);  is_recfun(r,a,H,f);  is_recfun(r,b,H,g) |]
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      ==> <x,a>:r \<longrightarrow> <x,b>:r \<longrightarrow> f`x=g`x"
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apply (frule_tac f = f in is_recfun_type)
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apply (frule_tac f = g in is_recfun_type)
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apply (simp add: is_recfun_def)
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apply (erule_tac a=x in wf_induct)
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apply (intro impI)
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apply (elim ssubst)
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apply (simp (no_asm_simp) add: vimage_singleton_iff restrict_def)
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apply (rule_tac t = "%z. H (?x,z) " in subst_context)
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apply (subgoal_tac "\<forall>y\<in>r-``{x}. \<forall>z. <y,z>:f \<longleftrightarrow> <y,z>:g")
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 apply (blast dest: transD)
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apply (simp add: apply_iff)
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apply (blast dest: transD intro: sym)
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done
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lemma is_recfun_cut:
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     "[| wf(r);  trans(r);
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         is_recfun(r,a,H,f);  is_recfun(r,b,H,g);  <b,a>:r |]
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      ==> restrict(f, r-``{b}) = g"
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apply (frule_tac f = f in is_recfun_type)
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apply (rule fun_extension)
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  apply (blast dest: transD intro: restrict_type2)
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 apply (erule is_recfun_type, simp)
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apply (blast dest: transD intro: is_recfun_equal)
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done
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subsection{*Recursion: Main Existence Lemma*}
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lemma is_recfun_functional:
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     "[| wf(r); trans(r); is_recfun(r,a,H,f); is_recfun(r,a,H,g) |]  ==>  f=g"
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by (blast intro: fun_extension is_recfun_type is_recfun_equal)
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lemma the_recfun_eq:
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    "[| is_recfun(r,a,H,f);  wf(r);  trans(r) |] ==> the_recfun(r,a,H) = f"
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apply (unfold the_recfun_def)
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apply (blast intro: is_recfun_functional)
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done
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(*If some f satisfies is_recfun(r,a,H,-) then so does the_recfun(r,a,H) *)
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lemma is_the_recfun:
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    "[| is_recfun(r,a,H,f);  wf(r);  trans(r) |]
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     ==> is_recfun(r, a, H, the_recfun(r,a,H))"
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by (simp add: the_recfun_eq)
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lemma unfold_the_recfun:
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     "[| wf(r);  trans(r) |] ==> is_recfun(r, a, H, the_recfun(r,a,H))"
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apply (rule_tac a=a in wf_induct, assumption)
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apply (rename_tac a1)
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apply (rule_tac f = "\<lambda>y\<in>r-``{a1}. wftrec (r,y,H)" in is_the_recfun)
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  apply typecheck
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apply (unfold is_recfun_def wftrec_def)
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  --{*Applying the substitution: must keep the quantified assumption!*}
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apply (rule lam_cong [OF refl])
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apply (drule underD)
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apply (fold is_recfun_def)
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apply (rule_tac t = "%z. H(?x,z)" in subst_context)
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apply (rule fun_extension)
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  apply (blast intro: is_recfun_type)
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 apply (rule lam_type [THEN restrict_type2])
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  apply blast
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 apply (blast dest: transD)
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apply atomize
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apply (frule spec [THEN mp], assumption)
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apply (subgoal_tac "<xa,a1> \<in> r")
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 apply (drule_tac x1 = xa in spec [THEN mp], assumption)
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apply (simp add: vimage_singleton_iff
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                 apply_recfun is_recfun_cut)
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apply (blast dest: transD)
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done
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subsection{*Unfolding @{term "wftrec(r,a,H)"}*}
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lemma the_recfun_cut:
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     "[| wf(r);  trans(r);  <b,a>:r |]
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      ==> restrict(the_recfun(r,a,H), r-``{b}) = the_recfun(r,b,H)"
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by (blast intro: is_recfun_cut unfold_the_recfun)
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(*NOT SUITABLE FOR REWRITING: it is recursive!*)
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lemma wftrec:
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    "[| wf(r);  trans(r) |] ==>
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          wftrec(r,a,H) = H(a, \<lambda>x\<in>r-``{a}. wftrec(r,x,H))"
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apply (unfold wftrec_def)
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apply (subst unfold_the_recfun [unfolded is_recfun_def])
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apply (simp_all add: vimage_singleton_iff [THEN iff_sym] the_recfun_cut)
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done
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subsubsection{*Removal of the Premise @{term "trans(r)"}*}
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(*NOT SUITABLE FOR REWRITING: it is recursive!*)
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lemma wfrec:
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    "wf(r) ==> wfrec(r,a,H) = H(a, \<lambda>x\<in>r-``{a}. wfrec(r,x,H))"
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apply (unfold wfrec_def)
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apply (erule wf_trancl [THEN wftrec, THEN ssubst])
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 apply (rule trans_trancl)
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apply (rule vimage_pair_mono [THEN restrict_lam_eq, THEN subst_context])
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 apply (erule r_into_trancl)
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apply (rule subset_refl)
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done
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(*This form avoids giant explosions in proofs.  NOTE USE OF == *)
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lemma def_wfrec:
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    "[| !!x. h(x)==wfrec(r,x,H);  wf(r) |] ==>
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     h(a) = H(a, \<lambda>x\<in>r-``{a}. h(x))"
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apply simp
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apply (elim wfrec)
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done
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lemma wfrec_type:
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    "[| wf(r);  a \<in> A;  field(r)<=A;
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        !!x u. [| x \<in> A;  u \<in> Pi(r-``{x}, B) |] ==> H(x,u) \<in> B(x)
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     |] ==> wfrec(r,a,H) \<in> B(a)"
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apply (rule_tac a = a in wf_induct2, assumption+)
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apply (subst wfrec, assumption)
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apply (simp add: lam_type underD)
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done
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lemma wfrec_on:
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 "[| wf[A](r);  a \<in> A |] ==>
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         wfrec[A](r,a,H) = H(a, \<lambda>x\<in>(r-``{a}) \<inter> A. wfrec[A](r,x,H))"
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apply (unfold wf_on_def wfrec_on_def)
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apply (erule wfrec [THEN trans])
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apply (simp add: vimage_Int_square cons_subset_iff)
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done
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text{*Minimal-element characterization of well-foundedness*}
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lemma wf_eq_minimal:
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     "wf(r) \<longleftrightarrow> (\<forall>Q x. x \<in> Q \<longrightarrow> (\<exists>z\<in>Q. \<forall>y. <y,z>:r \<longrightarrow> y\<notin>Q))"
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by (unfold wf_def, blast)
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end