We don't want to avoid proofs at `List.getIdx` and `Expr` when doing proofs by reflection.
The new encoding avoids that by using the fact that `vars` in
`Context` should never be empty.
To be honest, the best approach is still the old `unit`. We can just
rename it to `inhabitant` to make sure users don't assume it is the
unit of the AC operator. Then, we can just set it with the first element
of `vars` and avoid proofs at `denote`.
