The Minkowski difference of two multisets$S_1$ and $S_2$ is the multiset $S_1 \ominus S_2$ formed
by taking the collection of all possible differences $s_1 - s_2$, where $s_1$ is an element of $S_1$
and $s_2$ is an element of $S_2$. It is a natural generalization of the Minkowski sum.
