private function save_meta_data( $slug, $data ) {

		$prev = get_post_meta( $this->post->ID, $slug, true );

		if ( null === $data && $prev ) {
			delete_post_meta( $this->post->ID, $slug );
		} elseif ( $data !== $prev ) {
			update_post_meta( $this->post->ID, $slug, $data );
		}

	}