added serial_string;

--- a/src/Pure/library.ML	Sun Apr 30 22:50:05 2006 +0200
+++ b/src/Pure/library.ML	Sun Apr 30 22:50:06 2006 +0200
@@ -277,6 +277,7 @@
   val stamp: unit -> stamp
   type serial
   val serial: unit -> serial
+  val serial_string: unit -> string
   structure Object: sig type T end
 end;
 
@@ -1308,6 +1309,8 @@
 local val count = ref 0
 in fun serial () = inc count end;
 
+val serial_string = string_of_int o serial;
+
 
 (* generic objects *)