public function filter( $callback ) {

			$result = [];

			if ( is_callable( $callback ) ) {

				for ( $i=0; $i<$this->len; $i++ ) {
					if ( call_user_func( $callback, $this->items[$i], $i, $this ) ) {
						$result[] = &$this->items[$i];
					}
				}

			}

			$myName = get_class();

			return new $myName( $result );

		}