public function method($field, $methodName = null)
    {
        if (!$methodName) {
            $methodName = $field;
        }

        if (!is_string($field)) {
            throw new GenericException('Must be a string!');
        }
        if (!is_string($methodName)) {
            throw new GenericException('Must be a string!');
        }

        $this->fields[ $field ] = [
            'type'   => self::TYPE_METHOD,
            'method' => $methodName
        ];

        return $this;
    }