--- /dev/null Thu Jan 01 00:00:00 1970 +0000
+++ b/src/FOL/ex/Iff_Oracle.thy Mon Feb 16 20:15:40 2009 +0100
@@ -0,0 +1,76 @@
+(* Title: FOL/ex/Iff_Oracle.thy
+ Author: Lawrence C Paulson, Cambridge University Computer Laboratory
+ Copyright 1996 University of Cambridge
+*)
+
+header {* Example of Declaring an Oracle *}
+
+theory Iff_Oracle
+imports FOL
+begin
+
+subsection {* Oracle declaration *}
+
+text {*
+ This oracle makes tautologies of the form @{text "P <-> P <-> P <-> P"}.
+ The length is specified by an integer, which is checked to be even
+ and positive.
+*}
+
+oracle iff_oracle = {*
+ let
+ fun mk_iff 1 = Var (("P", 0), @{typ o})
+ | mk_iff n = FOLogic.iff $ Var (("P", 0), @{typ o}) $ mk_iff (n - 1);
+ in
+ fn (thy, n) =>
+ if n > 0 andalso n mod 2 = 0
+ then Thm.cterm_of thy (FOLogic.mk_Trueprop (mk_iff n))
+ else raise Fail ("iff_oracle: " ^ string_of_int n)
+ end
+*}
+
+
+subsection {* Oracle as low-level rule *}
+
+ML {* iff_oracle (@{theory}, 2) *}
+ML {* iff_oracle (@{theory}, 10) *}
+ML {* Thm.proof_of (iff_oracle (@{theory}, 10)) *}
+
+text {* These oracle calls had better fail. *}
+
+ML {*
+ (iff_oracle (@{theory}, 5); error "?")
+ handle Fail _ => warning "Oracle failed, as expected"
+*}
+
+ML {*
+ (iff_oracle (@{theory}, 1); error "?")
+ handle Fail _ => warning "Oracle failed, as expected"
+*}
+
+
+subsection {* Oracle as proof method *}
+
+method_setup iff = {*
+ Method.simple_args OuterParse.nat (fn n => fn ctxt =>
+ Method.SIMPLE_METHOD
+ (HEADGOAL (Tactic.rtac (iff_oracle (ProofContext.theory_of ctxt, n)))
+ handle Fail _ => no_tac))
+*} "iff oracle"
+
+
+lemma "A <-> A"
+ by (iff 2)
+
+lemma "A <-> A <-> A <-> A <-> A <-> A <-> A <-> A <-> A <-> A"
+ by (iff 10)
+
+lemma "A <-> A <-> A <-> A <-> A"
+ apply (iff 5)?
+ oops
+
+lemma A
+ apply (iff 1)?
+ oops
+
+end