50:  In C mode, can I show just the lines that will be left after #ifdef
 commands are handled by the compiler?
  
  M-x hide-ifdef-mode.  (This is a minor mode.)
  
  You may have to (load "hideif") first.  If you want to do this regularly,
  put this in your .emacs file:
  
    (autoload 'hide-ifdef-mode "hideif" nil t)
  
  {Yes, I know, this should be in lisp/loaddefs.el already.}