[Greasemonkey] accessing sub frames

[Vectorspace]

I can access the dom of a sub frame using frames[i] or 
document.getElementsByTagName("frame")[i].contentDocument but neither 
allows me to go another level down in the frame tree. What is the best 
way of accessing the sub frames of a page, and their sub frames, and 
their sub frames, and so on? I'd like to go down 3-4 levels.