code: private static int codeToUnary(final int code) {
      final int result;
  
      switch (code) {
        case CODE_MINUS:
          result = CODE_UNARYMINUS;
          break;
        case CODE_ADD:
          result = CODE_UNARYPLUS;
          break;
        default:
          result = code;
          break;
      }
      return result;
    }
docs: Encode code of an operator to code of similar unary operator.
  
  @param code a code of operator.
  @return code of an unary similar operator if it exists, the same code otherwise