let checkers = (side n.
pen color:none
repeat n (i.
right 90
# one row
repeat n (j.
if ((i+j)%2 == 0) then (
fill color:white
) else (
fill color:gray
)
square side
up (fw side)
)
# back to square number 1
up (
bk side*n
left 90
fw side
)
)
)
checkers 20 20