public function merge( $old, $new ) {
		if ( empty( $old ) ) {
			if ( empty( $new ) ) {
				return null;
			}
			return $this->condition( $new );
		} else if ( empty( $new ) ) {
			return $this->condition( $old );
		}

		return $this->mergeConditions( $this->condition( $old ), $this->condition( $new ) );
	}