added stack.ML;

--- a/src/Pure/General/ROOT.ML	Tue Sep 13 22:19:29 2005 +0200
+++ b/src/Pure/General/ROOT.ML	Tue Sep 13 22:19:30 2005 +0200
@@ -4,6 +4,7 @@
 Library of general tools --- prefer this over the 'Standard ML Library'.
 *)
 
+use "stack.ML";
 use "ord_list.ML";
 use "alist.ML";
 use "table.ML";