XOR'ed drawing ??

[Sergei Naumov <serge astro unc edu>]

Hi!
I need to have a cross-hair on my drawable area, and, I guess, I have
to draw the lines in a XOR'ed mode, so I get the picture that the cross
is moving on back.

How do I do this? Is there any mechanism for that or I have to figure
out myself what the pixels are?

Thanks.

	Sergei