public function mergeQueryAttribute( $old, $new ) {
		if ( $new === null ) {
			return $old;
		}

		if ( $old === null ) {
			return $new;
		}

		return function ( $query_vars ) use ( $old, $new ) {
			return call_user_func( $new, call_user_func( $old, $query_vars ) );
		};
	}