In `onDraw:' method it is possible paint things on the canvas.

Is it possible to erase (i.e. to make transparent) already drawn parts of canvas? I did not find out how.

Such mechanism would be useful also. I have been playing around a little bit:
	Silly things: http://altair.dcs.elf.stuba.sk/wiki/Kosik/Glyphs

Imagine a black square. Then imagine that you cut out a (smaller) circle from its middle so that you can see through that hole. Is it possible to write `drawOn:' which does that? It does not sound very complicated task, but I wasn't able to figure out how to accomplish it.

I am really curious.

Regards
-- 
mk