src/CCL/Gfp.ML
author paulson
Thu Sep 26 16:12:25 1996 +0200 (1996-09-26)
changeset 2035 e329b36d9136
parent 1459 d12da312eff4
child 3837 d7f033c74b38
permissions -rw-r--r--
Ran expandshort; used stac instead of ssubst
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(*  Title:      CCL/gfp
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    ID:         $Id$
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Modified version of
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    Title:      HOL/gfp
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    Author:     Lawrence C Paulson, Cambridge University Computer Laboratory
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    Copyright   1993  University of Cambridge
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For gfp.thy.  The Knaster-Tarski Theorem for greatest fixed points.
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*)
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open Gfp;
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(*** Proof of Knaster-Tarski Theorem using gfp ***)
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(* gfp(f) is the least upper bound of {u. u <= f(u)} *)
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val prems = goalw Gfp.thy [gfp_def] "[| A <= f(A) |] ==> A <= gfp(f)";
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by (rtac (CollectI RS Union_upper) 1);
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by (resolve_tac prems 1);
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qed "gfp_upperbound";
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val prems = goalw Gfp.thy [gfp_def]
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    "[| !!u. u <= f(u) ==> u<=A |] ==> gfp(f) <= A";
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by (REPEAT (ares_tac ([Union_least]@prems) 1));
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by (etac CollectD 1);
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qed "gfp_least";
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val [mono] = goal Gfp.thy "mono(f) ==> gfp(f) <= f(gfp(f))";
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by (EVERY1 [rtac gfp_least, rtac subset_trans, atac,
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            rtac (mono RS monoD), rtac gfp_upperbound, atac]);
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qed "gfp_lemma2";
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val [mono] = goal Gfp.thy "mono(f) ==> f(gfp(f)) <= gfp(f)";
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by (EVERY1 [rtac gfp_upperbound, rtac (mono RS monoD), 
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            rtac gfp_lemma2, rtac mono]);
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qed "gfp_lemma3";
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val [mono] = goal Gfp.thy "mono(f) ==> gfp(f) = f(gfp(f))";
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by (REPEAT (resolve_tac [equalityI,gfp_lemma2,gfp_lemma3,mono] 1));
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qed "gfp_Tarski";
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(*** Coinduction rules for greatest fixed points ***)
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(*weak version*)
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val prems = goal Gfp.thy
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    "[| a: A;  A <= f(A) |] ==> a : gfp(f)";
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by (rtac (gfp_upperbound RS subsetD) 1);
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by (REPEAT (ares_tac prems 1));
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qed "coinduct";
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val [prem,mono] = goal Gfp.thy
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    "[| A <= f(A) Un gfp(f);  mono(f) |] ==>  \
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\    A Un gfp(f) <= f(A Un gfp(f))";
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by (rtac subset_trans 1);
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by (rtac (mono RS mono_Un) 2);
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by (rtac (mono RS gfp_Tarski RS subst) 1);
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by (rtac (prem RS Un_least) 1);
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by (rtac Un_upper2 1);
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qed "coinduct2_lemma";
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(*strong version, thanks to Martin Coen*)
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val ainA::prems = goal Gfp.thy
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    "[| a: A;  A <= f(A) Un gfp(f);  mono(f) |] ==> a : gfp(f)";
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by (rtac coinduct 1);
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by (rtac (prems MRS coinduct2_lemma) 2);
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by (resolve_tac [ainA RS UnI1] 1);
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qed "coinduct2";
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(***  Even Stronger version of coinduct  [by Martin Coen]
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         - instead of the condition  A <= f(A)
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                           consider  A <= (f(A) Un f(f(A)) ...) Un gfp(A) ***)
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val [prem] = goal Gfp.thy "mono(f) ==> mono(%x.f(x) Un A Un B)";
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by (REPEAT (ares_tac [subset_refl, monoI, Un_mono, prem RS monoD] 1));
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qed "coinduct3_mono_lemma";
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val [prem,mono] = goal Gfp.thy
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    "[| A <= f(lfp(%x.f(x) Un A Un gfp(f)));  mono(f) |] ==> \
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\    lfp(%x.f(x) Un A Un gfp(f)) <= f(lfp(%x.f(x) Un A Un gfp(f)))";
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by (rtac subset_trans 1);
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by (rtac (mono RS coinduct3_mono_lemma RS lfp_lemma3) 1);
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by (rtac (Un_least RS Un_least) 1);
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by (rtac subset_refl 1);
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by (rtac prem 1);
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by (rtac (mono RS gfp_Tarski RS equalityD1 RS subset_trans) 1);
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by (rtac (mono RS monoD) 1);
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by (stac (mono RS coinduct3_mono_lemma RS lfp_Tarski) 1);
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by (rtac Un_upper2 1);
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qed "coinduct3_lemma";
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val ainA::prems = goal Gfp.thy
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    "[| a:A;  A <= f(lfp(%x.f(x) Un A Un gfp(f))); mono(f) |] ==> a : gfp(f)";
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by (rtac coinduct 1);
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by (rtac (prems MRS coinduct3_lemma) 2);
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by (resolve_tac (prems RL [coinduct3_mono_lemma RS lfp_Tarski RS ssubst]) 1);
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by (rtac (ainA RS UnI2 RS UnI1) 1);
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qed "coinduct3";
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(** Definition forms of gfp_Tarski, to control unfolding **)
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val [rew,mono] = goal Gfp.thy "[| h==gfp(f);  mono(f) |] ==> h = f(h)";
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by (rewtac rew);
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by (rtac (mono RS gfp_Tarski) 1);
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qed "def_gfp_Tarski";
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val rew::prems = goal Gfp.thy
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    "[| h==gfp(f);  a:A;  A <= f(A) |] ==> a: h";
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by (rewtac rew);
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by (REPEAT (ares_tac (prems @ [coinduct]) 1));
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qed "def_coinduct";
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val rew::prems = goal Gfp.thy
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    "[| h==gfp(f);  a:A;  A <= f(A) Un h; mono(f) |] ==> a: h";
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by (rewtac rew);
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by (REPEAT (ares_tac (map (rewrite_rule [rew]) prems @ [coinduct2]) 1));
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qed "def_coinduct2";
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val rew::prems = goal Gfp.thy
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    "[| h==gfp(f);  a:A;  A <= f(lfp(%x.f(x) Un A Un h)); mono(f) |] ==> a: h";
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by (rewtac rew);
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by (REPEAT (ares_tac (map (rewrite_rule [rew]) prems @ [coinduct3]) 1));
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qed "def_coinduct3";
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(*Monotonicity of gfp!*)
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val prems = goal Gfp.thy
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    "[| mono(f);  !!Z. f(Z)<=g(Z) |] ==> gfp(f) <= gfp(g)";
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by (rtac gfp_upperbound 1);
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by (rtac subset_trans 1);
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by (rtac gfp_lemma2 1);
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by (resolve_tac prems 1);
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by (resolve_tac prems 1);
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qed "gfp_mono";