src/Pure/display.ML

             prt_typ_no_tvars T, Pretty.str " ::", Pretty.brk 1, prt_sort S];
 
     val tfrees = map (fn v => TFree (v, []));
     fun pretty_type syn (t, (Type.LogicalType n, _)) =
           if syn then NONE
-          else SOME (prt_typ (Type (t, tfrees (Name.invent_list [] "'a" n))))
+          else SOME (prt_typ (Type (t, tfrees (Name.invents Name.context Name.aT n))))
       | pretty_type syn (t, (Type.Abbreviation (vs, U, syn'), _)) =
           if syn <> syn' then NONE
           else SOME (Pretty.block
             [prt_typ (Type (t, tfrees vs)), Pretty.str " =", Pretty.brk 1, prt_typ U])
       | pretty_type syn (t, (Type.Nonterminal, _)) =