Introduce `SingletonMS` class for multiset singletons.
- Define set-like notation
{[+ x1; ..; xn ]}
for multisets in terms of the new singleton class and disjoint union⊎
. - Remove
SemiSet
instance for multisets. - Prove lemmas regarding
∈
and∉
for multisets since we no longer get the generic versions for sets. - Provide
SetUnfoldElemOf
instances for multisets since we no longer get the generic versions for sets. - Prove lemmas new regarding
∈
and∉
for∩
Fixes #100 (closed), #98 (closed) and #87 (closed).
This MR is an alternative to !232 (closed).