Pretty-printing change to emphasize the scope of assumptions

--- a/src/Pure/Syntax/mixfix.ML	Thu Sep 05 10:29:52 1996 +0200
+++ b/src/Pure/Syntax/mixfix.ML	Thu Sep 05 10:30:42 1996 +0200
@@ -154,7 +154,7 @@
   ("_aprop",    "aprop => prop",                  Delimfix "PROP _"),
   ("",          "prop => asms",                   Delimfix "_"),
   ("_asms",     "[prop, asms] => asms",           Delimfix "_;/ _"),
-  ("_bigimpl",  "[asms, prop] => prop",           Mixfix ("((3[| _ |]) ==>/ _)", [0, 1], 1)),
+  ("_bigimpl",  "[asms, prop] => prop",           Mixfix ("((3[| _ |])/ ==> _)", [0, 1], 1)),
   ("_ofclass",  "[type, logic] => prop",          Delimfix "(1OFCLASS/(1'(_,/ _')))"),
   ("_K",        "'a",                             NoSyn),
   ("",          "id => logic",                    Delimfix "_"),