protected function clean( $meta = null ) {

    //
    if( $meta && !( $meta instanceof StorageMeta ) ) {
      $meta = $this->getMeta( $meta );
    }

    // clear the cache
    if( !$meta || empty( $meta->token ) ) $this->cache = [];
    else foreach( $this->cache as $i => $_ ) {

      if( empty( $i ) || $i == $meta->token[ 0 ] || strpos( $i, $meta->token[ 0 ] ) === 0 ) {
        unset( $this->cache[ $i ] );
      }
    }
  }