add cartesian product to set operator listing

[geemus]

When I got to the models/arbitrage example I was confused about what \X was, since it didn't appear to have been previously defined/explained. Hoping this might help.

[hwayne]

Kinda miffed at myself for forgetting to explain \X!

I think cartesian products are interesting enough that I'm going to add a paragraph on them in the "Tuples" section vs just putting them in the set operator chart. Thanks for bringing this up!

[geemus]

Sounds good. That seemed like maybe the best place to put it of places that existed, but I did worry it left out some important details. Breaking it out in more detail sounds great.