From: ldo@nz.invalid   
      
   Emacs has this feature called “overlays”, which let you set custom   
   display attributes for portions of a text buffer. This can include   
   completely hiding the text. That can be handy if you want to look at   
   two parts of a source file while temporarily ignoring some irrelevant   
   details in-between.   
      
   Here is a command that hides a selected region of text, replacing it   
   with a distinctive marker symbol, and also attaching a distinctive   
   category identifier for the overlay:   
      
   (defun hide_text (beg end)   
       (interactive "r")   
       (let   
           (   
               (olay (make-overlay beg end))   
           )   
           (overlay-put olay 'category 'collapsar)   
           (overlay-put olay   
               'display #("🐧\n" 0 1 (face '(:foreground "turquoise"   
   :background "yellow")))   
           )   
           (overlay-put olay 'evaporate t)   
           (deactivate-mark)   
       ) ; let   
   ) ; hide_text   
      
   Here is a function that invokes a specified callback for all overlays   
   of that category that overlap a specified position:   
      
   (defun foreach_hidden_text_at (act pos)   
       (dolist (olay (overlays-at pos))   
           (when (eq (overlay-get olay 'category) 'collapsar)   
               (funcall act olay)   
           ) ; when   
       ) ; dolist   
   ) ; foreach_hidden_text_at   
      
   And here is a command that uses foreach_hidden_text_at to reveal all   
   hidden text at the current position:   
      
   (defun reveal_text ()   
       (interactive)   
       (let   
           (   
               (revealed-something)   
           )   
           (foreach_hidden_text_at   
               (lambda (olay)   
                   (delete-overlay olay)   
                   (setq revealed-something t)   
               ) ; lambda   
               (point)   
           ) ; foreach_hidden_text_at   
           (unless revealed-something   
               (ding)   
           ) ; unless   
       ) ; let   
   ) ; reveal_text   
      
   --- SoupGate-Win32 v1.05   
    * Origin: you cannot sedate... all the things you hate (1:229/2)