let pp_token lexbuf fmt = function
| Id x -> Format.fprintf fmt "ident '%s'" x
| Key k -> Format.fprintf fmt "'%s'" k
| Proof _ -> Format.fprintf fmt "Proof...Qed"
| Eof -> Format.fprintf fmt "end-of-file"
| Word -> Format.fprintf fmt "start of '%s'" (Lexing.lexeme lexbuf)