Paint:
PROC [dc: Context] = {
[xd, yd] ← UserToWorld[dc, x, y];
[xl1, yl1] ← WorldToUser[dc, xd-3, yd-3];
[xl2, yl2] ← WorldToUser[dc, xd-2, yd-2];
[xu1, yu1] ← WorldToUser[dc, xd+3, yd+3];
[xu2, yu2] ← WorldToUser[dc, xd+2, yd+2];
[] ← Save[dc];
[] ← SetPaintMode[dc, opaque];
SetColor[dc, black];
DrawBox[dc, [xmax: xu1, ymax: yu1, xmin: xl1, ymin: yl1]];
SetColor[dc, white];
DrawBox[dc, [xmax: xu2, ymax: yu2, xmin: xl2, ymin: yl2]];
Restore[dc];
};