Try this in screen 13
a% = POINT (x%, y%)
IF a% > 31 AND a% < 183 THEN
PSET (x%, y%), a% + 72
ELSEIF a% < 32 AND a% > 14 THEN
PSET (x%, y%), a% + 1
ELSEIF a% > 182 THEN
PSET (x%, y%), 0
END IF
The colours 0 - 14 are oddballs you will have to assign shadows to them, but the ones above will be dead-on, this is using the default palettes for screen 13.