src/HOL/Induct/Comb.ML
 author berghofe Fri, 24 Jul 1998 13:39:47 +0200 changeset 5191 8ceaa19f7717 parent 5148 74919e8f221c child 5223 4cb05273f764 permissions -rw-r--r--
Renamed '\$' to 'Scons' because of clashes with constants of the same name in theories using datatypes.
```
(*  Title:      HOL/ex/comb.ML
ID:         \$Id\$
Author:     Lawrence C Paulson

Combinatory Logic example: the Church-Rosser Theorem
Curiously, combinators do not include free variables.

Example taken from
J. Camilleri and T. F. Melham.
Reasoning with Inductively Defined Relations in the HOL Theorem Prover.
Report 265, University of Cambridge Computer Laboratory, 1992.

HOL system proofs may be found in
/usr/groups/theory/hvg-aftp/contrib/rule-induction/cl.ml
*)

open Comb;

(*** Reflexive/Transitive closure preserves the Church-Rosser property
So does the Transitive closure; use r_into_trancl instead of rtrancl_refl
***)

val [_, spec_mp] = [spec] RL [mp];

(*Strip lemma.  The induction hyp is all but the last diamond of the strip.*)
Goalw [diamond_def]
"[| diamond(r);  (x,y):r^* |] ==> \
\         ALL y'. (x,y'):r --> (EX z. (y',z): r^* & (y,z): r)";
by (etac rtrancl_induct 1);
by (Blast_tac 1);
by (slow_best_tac (set_cs addIs [r_into_rtrancl RSN (2, rtrancl_trans)]
val diamond_strip_lemmaE = result() RS spec RS mp RS exE;

val [major] = goal Comb.thy "diamond(r) ==> diamond(r^*)";
by (rewtac diamond_def);  (*unfold only in goal, not in premise!*)
by (rtac (impI RS allI RS allI) 1);
by (etac rtrancl_induct 1);
by (Blast_tac 1);
by (slow_best_tac  (*Seems to be a brittle, undirected search*)
(set_cs addIs [r_into_rtrancl RSN (2, rtrancl_trans)]
qed "diamond_rtrancl";

(*Derive a case for each combinator constructor*)
val K_contractE = contract.mk_cases comb.simps "K -1-> z";
val S_contractE = contract.mk_cases comb.simps "S -1-> z";
val Ap_contractE = contract.mk_cases comb.simps "x#y -1-> z";

Goalw [I_def] "I -1-> z ==> P";
by (Blast_tac 1);
qed "I_contract_E";

Goal "K#x -1-> z ==> (EX x'. z = K#x' & x -1-> x')";
by (Blast_tac 1);
qed "K1_contractD";

Goal "x ---> y ==> x#z ---> y#z";
by (etac rtrancl_induct 1);
by (ALLGOALS (blast_tac (claset() addIs [r_into_rtrancl, rtrancl_trans])));
qed "Ap_reduce1";

Goal "x ---> y ==> z#x ---> z#y";
by (etac rtrancl_induct 1);
by (ALLGOALS (blast_tac (claset() addIs [r_into_rtrancl, rtrancl_trans])));
qed "Ap_reduce2";

(** Counterexample to the diamond property for -1-> **)

Goal "K#I#(I#I) -1-> I";
by (rtac contract.K 1);
qed "KIII_contract1";

Goalw [I_def] "K#I#(I#I) -1-> K#I#((K#I)#(K#I))";
by (Blast_tac 1);
qed "KIII_contract2";

Goal "K#I#((K#I)#(K#I)) -1-> I";
by (Blast_tac 1);
qed "KIII_contract3";

Goalw [diamond_def] "~ diamond(contract)";
by (blast_tac (claset() addIs [KIII_contract1,KIII_contract2,KIII_contract3]) 1);
qed "not_diamond_contract";

(*** Results about Parallel Contraction ***)

(*Derive a case for each combinator constructor*)
val K_parcontractE = parcontract.mk_cases comb.simps "K =1=> z";
val S_parcontractE = parcontract.mk_cases comb.simps "S =1=> z";
val Ap_parcontractE = parcontract.mk_cases comb.simps "x#y =1=> z";

(*** Basic properties of parallel contraction ***)

Goal "K#x =1=> z ==> (EX x'. z = K#x' & x =1=> x')";
by (Blast_tac 1);
qed "K1_parcontractD";

Goal "S#x =1=> z ==> (EX x'. z = S#x' & x =1=> x')";
by (Blast_tac 1);
qed "S1_parcontractD";

Goal "S#x#y =1=> z ==> (EX x' y'. z = S#x'#y' & x =1=> x' & y =1=> y')";
by (Blast_tac 1);
qed "S2_parcontractD";

(*The rules above are not essential but make proofs much faster*)

(*Church-Rosser property for parallel contraction*)
Goalw [diamond_def] "diamond parcontract";
by (rtac (impI RS allI RS allI) 1);
by (etac parcontract.induct 1 THEN prune_params_tac);
by Safe_tac;
by (ALLGOALS Blast_tac);
qed "diamond_parcontract";

(*** Equivalence of x--->y and x===>y ***)

Goal "contract <= parcontract";
by (rtac subsetI 1);
by (split_all_tac 1);
by (etac contract.induct 1);
by (ALLGOALS Blast_tac);
qed "contract_subset_parcontract";

(*Reductions: simply throw together reflexivity, transitivity and
the one-step reductions*)

(*Example only: not used*)
Goalw [I_def] "I#x ---> x";
by (Blast_tac 1);
qed "reduce_I";

Goal "parcontract <= contract^*";
by (rtac subsetI 1);
by (split_all_tac 1);
by (etac parcontract.induct 1 THEN prune_params_tac);
by (ALLGOALS Blast_tac);
qed "parcontract_subset_reduce";

Goal "contract^* = parcontract^*";
by (REPEAT
(resolve_tac [equalityI,
contract_subset_parcontract RS rtrancl_mono,
parcontract_subset_reduce RS rtrancl_subset_rtrancl] 1));
qed "reduce_eq_parreduce";

Goal "diamond(contract^*)";
by (simp_tac (simpset() addsimps [reduce_eq_parreduce, diamond_rtrancl,
diamond_parcontract]) 1);
qed "diamond_reduce";

writeln"Reached end of file.";
```