XDI graph as XRIs

Hey Bill,

I'm sitting together with Nat and Tatsuki, talking about various XDI topics. One issue that came up was the following: I think it was you who suggested a few times that every XDI document can be expressed as a simple list of XRIs, right?

For example, if I have this XDI graph:

=markus
   +friend
      =bill.barnhill
      =drummond

I could just express it using these XRIs:

=markus/+friend/=bill.barnhill
=markus/+friend/=drummond

Right?

Now the question is, how does that work with literals? If I have this:

=markus
   +email
      ""

Then what's the XRI that represents this statement? I'm sure someone has thought about that before, but I don't really remember how it works or if it works at all?

Markus