Rather than doing set compatibility calculation purely in Multiset implementations provided a method on the ISet interface which supports this feature. This allows the implementation to be easily improved without having to change code in multiple places.
Potentially current set compatibility calculations could be speeded up by inclusion of fast hash code (i.e. integer) equality comparisons since set compatibility is always done on node equality which can often be costly to compute. Obviously once the initial refactor is done then this change could be attempted and benchmarked