made SML/NJ happy

--- a/src/HOL/Tools/SMT/z3_proof_tools.ML	Tue Nov 08 10:48:58 2011 +0100
+++ b/src/HOL/Tools/SMT/z3_proof_tools.ML	Tue Nov 08 14:29:24 2011 +0100
@@ -74,10 +74,11 @@
     in (case select ct of [] => select' ct | xthms' => xthms') end
 in
 
-val net_instances =
+fun net_instances net =
   instances_from_net false (fn f => fn (i, thm) => Option.map (pair i) (f thm))
+    net
 
-val net_instance = try hd oo instances_from_net true I
+fun net_instance net = try hd o instances_from_net true I net
 
 end