classes.singleton: fix "summary" of singletons. Fixes #145.

core/classes/singleton/singleton.factor

@@ -1,8 +1,8 @@
 ! Copyright (C) 2008, 2010 Doug Coleman, Slava Pestov.
 ! See http://factorcode.org/license.txt for BSD license.
 USING: classes classes.algebra classes.algebra.private
-classes.predicate classes.predicate.private kernel sequences
-words ;
+classes.predicate classes.predicate.private definitions kernel
+sequences words ;
 IN: classes.singleton
 
 <PRIVATE
@@ -26,3 +26,6 @@ M: singleton-class (classes-intersect?)
 
 M: singleton-class predicate-quot
     singleton-predicate-quot ;
+
+M: singleton-class definer
+    drop \ SINGLETON: f ;