function hasNotStartedSerie($user, $serie)
{
    if (isEmptySeries($serie)) {
        return false;
    }
    $exercises = getAllExercisesOfSeries($serie);
    foreach ($exercises as $exercise) {
        if (getAccomplishedExercise($user, $exercise)) {
            return false;
        }
    }
    return true;
}
function returnSeriesRandom($serie)
{
    $result = DB::select(' SELECT *
                        FROM series
                        WHERE series.id != ?
                        ORDER BY RAND()
                        limit 2', [$serie->id]);
    if (empty($result)) {
        return returnSeriesRandom($serie);
    }
    if (isEmptySeries($result[0])) {
        return returnSeriesRandom($serie);
    }
    return $result;
}