Emacs: Toggle Elfeed search filter to use "+unread"

elfeed-search-toggle-unread-filter.el

  (defun rytswd/elfeed-search-toggle-unread-filter ()
    "Update the current Elfeed search filter to toggle \"+unread\"."
    (interactive)
    (let* ((unread-tag "+unread")
           (filter-items (split-string elfeed-search-filter))
           (has-unread (member unread-tag filter-items))
           (filter-items-no-unread (remove unread-tag filter-items)))
      (when (not has-unread)
        (nconc filter-items-no-unread (list unread-tag)))
      (setq elfeed-search-filter (mapconcat 'identity filter-items-no-unread " "))
      (elfeed-search-update--force)
      ))