[lukemerrick]

I actually played with Unityper.jl and SumTypes.jl, but my conclusion was that if I was going to depart from dispatch on Julia types in my code, I might as well just stick to an untyped tree, since either way I'd have to have a single `evaluate` function for interpreting any kind of node.

Reconsidering now, it seems that there might be benefits beyond type dispatch to having a typed syntax tree, so maybe I'll give that a shot as a next step!