protected function indexedMarksToArray(array $givenMarks)
    {

        $marks = [];

        foreach ($givenMarks as $mark) {

            if (strpos($mark, '!') === 0) {
                $marks[substr($mark, 1)] = false;
                continue;
            }

            $marks[$mark] = true;

        }

        return $marks;
    }