public function setSubCat($oCat, $sKey = null)
    {
        if ($sKey) {
            $this->_aSubCats[$sKey] = $oCat;
        } else {
            $this->_aSubCats[] = $oCat;
        }

        // keeping ref. to parent
        $oCat->setParentCategory($this);

        if ($oCat->getIsVisible()) {
            $this->setHasVisibleSubCats(true);
        }
    }