made SML/NJ happier

--- a/src/HOL/Tools/ATP/atp_proof_reconstruct.ML	Mon Dec 16 23:36:54 2013 +0100
+++ b/src/HOL/Tools/ATP/atp_proof_reconstruct.ML	Tue Dec 17 09:42:38 2013 +0100
@@ -570,7 +570,7 @@
         fun in_group group = member (op =) group o hd
         fun group_of sko = the (find_first (fn group => in_group group sko) groups)
 
-        fun new_step group skoss_steps =
+        fun new_step group (skoss_steps : ('a * (term, 'b) atp_step) list) =
           let
             val t =
               skoss_steps