Functionality for annotating each expression with its Scope or reduced form

[expipiplus1]

It would be super useful to be able to annotate each expression with its scope. This would allow one to arbitrarily reduce/evaluate parts of the expression tree.

I'm sure that all the building blocks are here for this. At the moment scope generation is quite intertwined with evaluation/reduction, which as far as I can tell makes it a little tricky to get exactly what I want here.