src/Pure/Syntax/mixfix.ML

   ("_bracket", "[types, type] => type", Mixfix ("([_]/ \\<Rightarrow> _)", [0, 0], 0)),
   ("_ofsort",  "[tid, sort] => type",   Mixfix ("_\\<Colon>_", [SynExt.max_pri, 0], SynExt.max_pri)),
   ("_constrain", "['a, type] => 'a",    Mixfix ("_\\<Colon>_", [4, 0], 3)),
   ("_idtyp",   "[id, type] => idt",     Mixfix ("_\\<Colon>_", [], 0)),
   ("_lambda",  "[pttrns, 'a] => logic", Mixfix ("(3\\<lambda>_./ _)", [0, 3], 3)),
-  ("==",       "['a::{}, 'a] => prop",  InfixrName ("\\<equiv>", 2)),
+  ("==",       "['a, 'a] => prop",      InfixrName ("\\<equiv>", 2)),
   ("!!",       "[idts, prop] => prop",  Mixfix ("(3\\<And>_./ _)", [0, 0], 0)),
   ("==>",      "[prop, prop] => prop",  InfixrName ("\\<Longrightarrow>", 1)),
-  ("=?=",      "['a::{}, 'a] => prop",  InfixrName ("\\<equiv>\\<^sup>?", 2)),
   ("_DDDOT",   "aprop",                 Delimfix "\\<dots>"),
   ("_bigimpl", "[asms, prop] => prop",  Mixfix ("((1\\<lbrakk>_\\<rbrakk>)/ \\<Longrightarrow> _)", [0, 1], 1)),
   ("_DDDOT",   "logic",                 Delimfix "\\<dots>")];
 
 end;