function ppmenu()
 
  $Id: ppmenu.m,v 1.75 2003-09-23 22:39:39-05 brinkman Exp $
  (c) 2001, Peter Brinkmann (brinkman@math.uiuc.edu)
 
  ppmenu: text menu for df and friends.
 
  Usage: ppmenu;
 
  Note: The menu allows the user to change the names of independent and
  dependent variables. In order to avoid conflicts between those names
  and the names of internal variables, we adopt the convention that user
  defined variable names can only have one letter, whereas internal
  variable names have to have at least two letters.
 
  Acknowledgments:
 	- Many thanks to Robert Jerrard and Richard Laugesen for valuable
 	  feedback and suggestions.