An implementation of `select*` must call `next-fn` on each
subvalue of `structure`. The result of `select*` is specified
as follows:

1. `NONE` if `next-fn` never called
2. `NONE` if all calls to `next-fn` return `NONE`
3. Otherwise, any non-`NONE` return value from calling `next-fn`

An implementation of `transform*` must use `next-fn` to transform
any subvalues of `structure` and then merge those transformed values
back into `structure`. Everything else in `structure` must be unchanged.