public function addKeyword($keyword, array $properties = null)
    {
        if ($properties === null) {
            $properties = [];
        }

        foreach ($properties as $propertyIdent) {
            $val = ('%'.$keyword.'%');
            $this->addFilter([
                'property' => $propertyIdent,
                'operator' => 'LIKE',
                'value'    => $val,
                'operand'  => 'OR'
            ]);
        }

        return $this;
    }