for x: 0 .. maxx by 20

    drawline(x,0,midx,midy,black)
end for

for y: 0.. maxy by 20
    drawline (0, y, midx, midy, black)
    end for

for i: 0.. maxy by 20
    drawline (maxx, i, midx, midy, black)
    end for

    for j: 0.. maxx by 20
    drawline (j, maxy, midx, midy, black)
    end for