changed precedences of _constrain (i.e. "::")

--- a/src/Pure/Syntax/mixfix.ML	Thu Oct 06 18:40:18 1994 +0100
+++ b/src/Pure/Syntax/mixfix.ML	Sat Oct 08 13:45:40 1994 +0100
@@ -166,6 +166,6 @@
   ("",          "id => logic",                    Delimfix "_"),
   ("",          "var => logic",                   Delimfix "_"),
   ("_appl",     "[logic, args] => logic",         Mixfix ("(1_/(1'(_')))", [max_pri, 0], max_pri)),
-  ("_constrain", "[logic, type] => logic",        Mixfix ("_::_", [max_pri, 0], 999))]
+  ("_constrain", "[logic, type] => logic",        Mixfix ("_::_", [4, 0], 3))]
 
 end;