Hasta La Motion!

What is the workflow for getting deep images out of Maya using Mental Ray and into Nuke for compositing? I quite like to know how to use the Position pass to create a custom depth pass in Nuke? Such as the operations and such.

From the Sigagraph paper on deep image compositing it says they use an un-over, what is an un-over operation in Nuke?

Any information would be very useful, as this technique will be quite handy for compositing volumetric fog, something that I need to do.

Here is that paper on deep image compositing, its very short and brief. Full credit goes to the authors.
http://www.johannessaam.com/deepImage.pdf

Thanks.