decode: qualified is always const;

--- a/src/Pure/type.ML	Fri Oct 10 14:51:58 1997 +0200
+++ b/src/Pure/type.ML	Fri Oct 10 15:44:48 1997 +0200
@@ -838,7 +838,7 @@
       | decode (t $ u) = decode t $ decode u
       | decode (Free (x, T)) =
           let val c = map_const x in
-            if is_const c then Const (c, certT T)
+            if is_const c orelse NameSpace.qualified c then Const (c, certT T)
             else if T = dummyT then Free (x, get_type (x, ~1))
             else constrain (Free (x, certT T)) (get_type (x, ~1))
           end