self_retrace :: Instructions -> Instructions -> Instructions -> Instructions -> Instructions
self_retrace Stop prev_colour prev_style prev_pen = Stop
self_retrace (PenDown i) prev_colour prev_style prev_pen = andThen (self_retrace i prev_colour prev_style (PenDown Stop)) Stop
self_retrace (PenUp i) prev_colour prev_style prev_pen = andThen (self_retrace i prev_colour prev_style (PenUp Stop)) Stop
self_retrace (SetColour x i) prev_colour prev_style prev_pen = andThen (self_retrace i (SetColour x Stop) prev_style prev_pen) Stop
self_retrace (SetStyle x i) prev_colour prev_style prev_pen = andThen (self_retrace i prev_colour (SetStyle x Stop) prev_pen) Stop
self_retrace (Move x i) prev_colour prev_style prev_pen = andThen prev_colour (andThen prev_style (andThen prev_pen (andThen (self_retrace i prev_colour prev_style prev_pen) (Move (-x) Stop))))
self_retrace (Turn x i) prev_colour prev_style prev_pen = andThen (self_retrace i prev_colour prev_style prev_pen) (Turn (-x) Stop)