code: static String opToStrNoFail(Token operator) {
      String res = opToStr(operator);
      if (res == null) {
        throw new Error("Unknown op " + operator);
      }
      return res;
    }
docs: Converts an operator's token value (see {@link Token}) to a string
  representation or fails.
  
  @param operator the operator's token value to convert
  @return the string representation
  @throws Error if the token value is not an operator