improving function is_iterable in quickcheck

--- a/src/Tools/quickcheck.ML	Mon Nov 22 11:35:02 2010 +0100
+++ b/src/Tools/quickcheck.ML	Mon Nov 22 11:35:06 2010 +0100
@@ -205,8 +205,9 @@
    end
 
 (* we actually assume we know the generators and its behaviour *)
-fun is_iteratable "small" = false
+fun is_iteratable "SML" = true
   | is_iteratable "random" = true
+  | is_iteratable _ = false
   
 fun test_term ctxt generator_name t =
   let