public function isEqual(Collection $collection, $compareFunction) {
    // ungleiche Anzahl ist nie gleich => fastcheck
    if (count($this) !== count($collection)) return FALSE;

    if (count($this->insertDiff($collection, $compareFunction)) > 0) return FALSE;
    if (count($this->deleteDiff($collection, $compareFunction)) > 0) return FALSE;
    
    return TRUE;
  }