public static function formDeleteAll() {

		if ( ! empty( $_POST['delete_all_covers'] ) && check_admin_referer( 'pb-delete-all-covers' ) ) {
			\Pressbooks\Utility\truncate_exports( 0, static::getCoversFolder() );
			delete_transient( 'dirsize_cache' ); /** @see get_dirsize() */
		}

		\Pressbooks\Redirect\location( admin_url( 'admin.php?page=pressbooks_cg' ) );
	}