private static function prependOperator($value, $operator = null)
    {

        if ($operator) {
            $operator = StringHelper::toLowerCase($operator);

            if (in_array($operator, static::$operators) || $operator === 'not') {
                if (is_array($value)) {
                    $values = [];

                    foreach ($value as $v) {
                        $values[] = $operator . ($operator === 'not' ? ' ' : '') . $v;
                    }

                    return $values;
                }

                return $operator . ($operator === 'not' ? ' ' : '') . $value;
            }
        }

        return $value;
    }