penreverse  -- Command, no inputs.  Abbreviation: px
     This command tells the display turtle to lower  its  "reversing  pen";
     thereafter,  when  the turtle moves, it turns on any points which were
     off, and turns off any points which were on.  The commands penup, pen-
     down,  penerase,  and penreverse are mutually exclusive; whichever was
     most recently used is  the  one  which  affects  the  turtle.   (Note:
     Graphics  terminals which cannot penreverse will treat this command as
     pendown.)
