Replaced "sum" (only usage?) by foldl op+

--- a/src/Pure/Thy/thm_database.ML	Fri Nov 01 15:39:37 1996 +0100
+++ b/src/Pure/Thy/thm_database.ML	Fri Nov 01 15:41:09 1996 +0100
@@ -202,7 +202,9 @@
       fun substsize(prop, tsig) =
             let val Some pat = extract prop
                 val (_,subst) = Pattern.match tsig (pat,obj)
-            in sum(map (fn (_,t) => size_of_term t) subst) end
+            in foldl op+
+		(0, map (fn (_,t) => size_of_term t) subst)
+            end
 
       fun thm_order ((p0:int,s0:int,_),(p1,s1,_)) =
             (((p0=0 andalso p1=0) orelse (p0<>0 andalso p1<>0)) andalso s0<=s1)