public function addIndexEngineIndex(ChildIndexEngineIndex $l)
    {
        if ($this->collIndexEngineIndices === null) {
            $this->initIndexEngineIndices();
            $this->collIndexEngineIndicesPartial = true;
        }

        if (!in_array($l, $this->collIndexEngineIndices->getArrayCopy(), true)) { // only add it if the **same** object is not already associated
            $this->doAddIndexEngineIndex($l);
        }

        return $this;
    }