[Groff] Cascading boxes with pic?

[Jeronimo Pellegrini]

Hello,

Ijust started learning to use pic but there's something I didn't
find in the tutorials I've found (and it looks like the section
of the manual on pic is empty).

I'd like to draw a number of boxes in "cascading" style, like this:

.PS
A:box wid 0.7i ht 1i
B:box wid 0.7i ht 1i at (A.e.x-0.1i, A.e.y-0.1i)
C:box wid 0.7i ht 1i at (B.e.x-0.1i, B.e.y-0.1i)
.PE

But this actually draws all of them completely -- I'd like the
topmost to actually hide part of the box below it, and so on, so
the picture would look as paper sheets diagonally aligned on a desk.

Is there a simple way to do that? Can I, for example, fill the boxes
with white so that'd cover the lines behind it?

Thanks,
Jeronimo