Added recursion equations for foldl to list_ss.

--- a/List.ML	Fri Mar 17 15:48:55 1995 +0100
+++ b/List.ML	Mon Mar 27 18:30:04 1995 +0200
@@ -37,6 +37,7 @@
    flat_Nil, flat_Cons,
    list_all_Nil, list_all_Cons,
    filter_Nil, filter_Cons,
+   foldl_Nil, foldl_Cons,
    length_Nil, length_Cons];