gnus: Store custom flags and keywords

 
 9.18.4 Store custom flags and keywords
 --------------------------------------
 
 The registry lets you set custom flags and keywords per message.  You
 can use the Gnus->Registry Marks menu or the ‘M M x’ keyboard shortcuts,
 where ‘x’ is the first letter of the mark’s name.
 
  -- Variable: gnus-registry-marks
      The custom marks that the registry can use.  You can modify the
      default list, if you like.  If you do, you’ll have to exit Emacs
      before they take effect (you can also unload the registry and
      reload it or evaluate the specific macros you’ll need, but you
      probably don’t want to bother).  Use the Customize interface to
      modify the list.
 
      By default this list has the ‘Important’, ‘Work’, ‘Personal’,
      ‘To-Do’, and ‘Later’ marks.  They all have keyboard shortcuts like
      ‘M M i’ for Important, using the first letter.
 
  -- Function: gnus-registry-mark-article
      Call this function to mark an article with a custom registry mark.
      It will offer the available marks for completion.
 
    You can use ‘defalias’ to install a summary line formatting function
 that will show the registry marks.  There are two flavors of this
 function, either showing the marks as single characters, using their
 ‘:char’ property, or showing the marks as full strings.
 
      ;; show the marks as single characters (see the :char property in
      ;; 'gnus-registry-marks'):
      ;; (defalias 'gnus-user-format-function-M 'gnus-registry-article-marks-to-chars)
 
      ;; show the marks by name (see 'gnus-registry-marks'):
      ;; (defalias 'gnus-user-format-function-M 'gnus-registry-article-marks-to-names)