public function allowEmptyFor($field, $flags, $when = true, $message = null)
    {
        $this->field($field)->allowEmpty($when);
        if ($message) {
            $this->_allowEmptyMessages[$field] = $message;
        }
        if ($flags !== null) {
            $this->_allowEmptyFlags[$field] = $flags;
        }

        return $this;
    }