(* Lexer function *)
let rec token buf =
  match%sedlex buf with
  | number ->
      let n = int_of_string (Sedlexing.Utf8.lexeme buf) in
      Some (Number n, buf)
  | op_plus ->
      Some (Op_plus, buf)
  | Plus xml_blank ->
      token buf
  | eof ->
      None
  | _ ->
      assert false