I think I'm a little confused. Why wouldn't you have just added the "roman" possibility from the get go? Could you provide an example of a situation when you don't know the universe of possibilities in advance?
If the user adds one, or new data enters from an outside source (pub/sub, sockets, etc)
Alright, this is a great point. It would be trivial for me to add addPossibilities. I think I'll do that! Thanks. This will really help for using PourOver to power tagging selection fields where you can ... add possibilities!
Exactly what @rattray said. It's something you get nearly for free in data-binding world and is much harder to accomplish in imperative code. I wish you luck, and make sure you blog about it when it's done! (I want to see how it's implemented.)