Why no SortedMultiset in Google Collections?

Google Collections contains the `Multiset` interface and the `TreeMultiset` class, but I was surprised to find that there is no corresponding `SortedMultiset` interface. Something like that would be very useful for modelling discrete probability distributions. Before I attempt to implement it myself, I would like to know if there is a specific reason for leaving it out, e.g. likely violation of `Multiset` or `Collection` invariants, or inherent performance problems etc. Edit: I didn't realise it originally but this is actually 3 separate requests: - A change to the return type of one method (`TreeMultiset.entrySet`) - An new interface to match the existing functionality of `TreeMultiset` - A new pair of methods to sum the counts in branches of the tree