Proof by guessing.

--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/src/HOL/ex/Guess.thy	Tue Jul 04 19:49:56 2006 +0200
@@ -0,0 +1,31 @@
+(*
+    ID:         $Id$
+    Author:     Makarius
+*)
+
+header {* Proof by guessing *}
+
+theory Guess
+imports Main
+begin
+
+lemma True
+proof
+
+  have 1: "\<exists>x. x = x" by simp
+
+  from 1 guess ..
+  from 1 guess x ..
+  from 1 guess x :: 'a ..
+  from 1 guess x :: nat ..
+
+  have 2: "\<exists>x y. x = x & y = y" by simp
+  from 2 guess apply - apply (erule exE conjE)+ done
+  from 2 guess x apply - apply (erule exE conjE)+ done
+  from 2 guess x y apply - apply (erule exE conjE)+ done
+  from 2 guess x :: 'a and y :: 'b apply - apply (erule exE conjE)+ done
+  from 2 guess x y :: nat apply - apply (erule exE conjE)+ done
+
+qed
+
+end