A running digest of a set of bytes, where elements can be added and removed.

Note that it's the caller's responsibility to ensure that the collection defined by the sequence of additions/removals is really a set. In particular: 1. the digest accepts a call to remove before the corresponding call to add 2. the digest will change if the same element is added twice. Note, however, that the digest rolls over if you add an element 2^{16 times; i.e., taking a digest d, then adding the same element 2}16 times results in d again.