fix subtle type inference bug in Metis -- different occurrences of the same Skolem might need to be typed differently, using paramify_vars overconstraints the typing problem

--- a/src/HOL/Tools/Metis/metis_translate.ML	Thu Jul 14 15:14:38 2011 +0200
+++ b/src/HOL/Tools/Metis/metis_translate.ML	Thu Jul 14 15:14:38 2011 +0200
@@ -94,7 +94,7 @@
   map_aterms (fn t as Const (s, _) =>
                  if String.isPrefix old_skolem_const_prefix s then
                    AList.lookup (op =) old_skolems s |> the
-                   |> map_types Type_Infer.paramify_vars
+                   |> map_types (map_type_tvar (K dummyT))
                  else
                    t
                | t => t)