This fixes #13 (closed). Now ∪ and ∩ are the usual multiset union and intersection (i.e. taking max and min, respectively), whereas ⊎ is the "sum" (i.e. adding the multiplicities).
Some questions/remarks:
⊎ and ∪ have different heights in my font (see ⊎∪). Is there maybe a better disjoint union symbol in unicode?∪, ∩, and ⊎, like distributivity. I did not attempt to prove a comprehensive set of such laws.∖ is still the operation that subtracts multiplicity. I think that's fine, since there is no sensible difference operator for multisets that matches up with the union operator.