src/Pure/Isar/outer_keyword.ML

   prf_chain, prf_decl, prf_asm, prf_asm_goal, prf_script] |> map kind_of;
 
 
 (* tags *)
 
-fun update_tags t ts = t :: remove (op =) t ts;
+fun update_tags t ts = t :: remove (op = : string * string -> bool) t ts;
 
 fun tag t (Keyword (s, ts)) = Keyword (s, update_tags t ts);
 fun tags_of (Keyword (_, ts)) = ts;
 
 val tag_theory = tag "theory";