An ordered collection of statements representing the context surrounding the current focus of our program transformation. That is, since some transformations work from the bottom up, we need to keep track of the statements we passed along the way when reaching for the bottom.

The tail of the list takes scope over the head of the list. Thus, the back/end of the list is towards the top of the program, whereas the front of the list is towards the bottom.

This type was formerly called Heap (presumably due to the Statement type being called Binding) but that seems like a misnomer to me since this really has nothing to do with allocation. However, it is still like a heap inasmuch as it's a dependency graph and we may wish to change the topological sorting or remove "garbage" (subject to correctness criteria).

TODO: Figure out what to do with SWeight, SGuard, SStuff, etc, so that we can use an IntMap (Statement abt) in order to speed up the lookup times in select. (Assuming callers don't use unsafePush unsafely: we can recover the order things were inserted from their varID since we've freshened them all and therefore their IDs are monotonic in the insertion order.)

Run a computation in the Expect monad, residualizing out all the statements in the final evaluation context. The second argument should include all the terms altered by the Eval expression; this is necessary to ensure proper hygiene; for example(s):

runExpect (pureEvaluate e) [Some2 e]

We use Some2 on the inputs because it doesn't matter what their type or locally-bound variables are, so we want to allow f to contain terms with different indices.