  This article describes lossless compression algorithms for multisets of
sequences, taking advantage of the multiset's unordered structure. Multisets
are a generalisation of sets where members are allowed to occur multiple times.
A multiset can be encoded na\"ively by simply storing its elements in some
sequential order, but then information is wasted on the ordering. We propose a
technique that transforms the multiset into an order-invariant tree
representation, and derive an arithmetic code that optimally compresses the
tree. Our method achieves compression even if the sequences in the multiset are
individually incompressible (such as cryptographic hash sums). The algorithm is
demonstrated practically by compressing collections of SHA-1 hash sums, and
multisets of arbitrary, individually encodable objects.
