private @Nullable ZenCodingNode parseMul(@Nullable ZenCodingNode expression) {
    ZenCodingToken operationToken = getToken();
    if (expression != null && operationToken instanceof OperationToken && ((OperationToken)operationToken).getSign() == '*') {
      advance();
      ZenCodingToken numberToken = getToken();
      if (numberToken instanceof NumberToken) {
        advance();
        return new MulOperationNode(expression, ((NumberToken)numberToken).getNumber());
      }
      return new UnaryMulOperationNode(expression);
    }
    return expression;
  }
private @Nullable ZenCodingNode parseMul(@Nullable ZenCodingNode expression)