BTrees are able to perform O(log(n)) lookups for the n-th largest/smallest element (a, b), but currently I believe that
BTreeSet don’t provide an API to leverage that, instead having to rely on in-order traversal via
.iter().nth(n), which will be O(n).
I propose adding a new O(log(n)) API along the lines of
BTreeSet, or specializing
.nth() for the relevant iterators (or both).