public function pushStorage( ezcCacheStackStorageConfiguration $storageConf )
    {
        if ( isset( $this->storageIdMap[$storageConf->id] ) )
        {
            throw new ezcCacheStackIdAlreadyUsedException(
                $storageConf->id
            );
        }
        if ( in_array( $storageConf->storage, $this->storageIdMap, true ) )
        {
            throw new ezcCacheStackStorageUsedTwiceException(
                $storageConf->storage
            );
        }
        array_unshift( $this->storageStack, $storageConf );
        $this->storageIdMap[$storageConf->id] = $storageConf->storage;
    }