Added {x.x=a} = a to !simpset.

--- a/src/HOL/Set.ML	Fri Jul 25 14:31:48 1997 +0200
+++ b/src/HOL/Set.ML	Fri Aug 01 09:39:28 1997 +0200
@@ -415,6 +415,10 @@
     
 AddSDs [singleton_inject];
 
+goal Set.thy "{x.x=a} = {a}";
+by(Blast_tac 1);
+qed "singleton_conv";
+Addsimps [singleton_conv];
 
 section "The universal set -- UNIV";