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,
     pendown, 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.)