[munk-a]

It's been a while since relational algebra so I don't feel confident saying "Yes" or "No" (I guess you could say my reply is Null) - but that seems like a really good base approach - especially having a state for being uncertain whether something is nullable or not... having an operator embeded fallback approach also seems to be a lot more usable for the developer.

Also, this is an aside, but is your thing named after Snake Plissken?

[default-kramer]

Yeah, kind of. I have seen Escape From New York but I don't really remember it. I was more aware of the character via the reference/homage in Metal Gear Solid 2. In any case, I don't love the name but the obvious choice (Racquel) was already taken and "Plisqin" had zero Google results so I said "meh, good enough."