public function add_to_feed( array $vars ) : array {

		# If it's not a feed, we're not interested:
		if ( ! isset( $vars['feed'] ) ) {
			return $vars;
		}

		if ( ! isset( $vars['post_type'] ) ) {
			$vars['post_type'] = [
				'post',
				$this->post_type,
			];
		} elseif ( is_array( $vars['post_type'] ) && ( count( $vars['post_type'] ) > 1 ) ) {
			$vars['post_type'][] = $this->post_type;
		}

		return $vars;

	}