### Multisets (bags)
[ Type multisets Funcon multiset Funcon multiset-elements Funcon multiset-occurrences Funcon multiset-insert Funcon multiset-delete Funcon is-submultiset ] Meta-variables GT <: ground-values Built-in Type multisets(GT)
*/ Built-in Funcon multiset(_:(GT)*) : =>multisets(GT)
/* Note that `multiset(...)` is not a constructor operation. The order of
argument values is ignored, but duplicates are significant, e.g.,
`multiset(1, 2, 2)` is equivalent to `multiset(2, 1, 2)`, but not to
`multiset(1, 2)` or `multiset(2, 1)`.
*/ Built-in Funcon multiset-elements(_:multisets(GT)) : =>(GT)*
/* For each multiset `MS`, the sequence of values `V*` returned by
`multiset-elements(MS)` contains each element of `MS` the same number of times
as `MS` does.
The order of the values in `V*` is unspecified, and may vary between multisets.
*/
Assertmultiset(multiset-elements(S)) == S
Built-in Funcon multiset-occurrences(_:GT, _:multisets(GT)) : =>natural-numbers
/* `multiset-occurrences(GV, MS)` returns the number of occurrences of `GV`
in `MS`.
*/ Built-in Funcon multiset-insert(_:GT, _:natural-numbers, _:multisets(GT)) : =>multisets(GT)
/* `multiset-insert(GV, N, MS)` returns the multiset that differs from `MS`
by containing `N` more copies of `GV`.
*/ Built-in Funcon multiset-delete(_:multisets(GT), _:GT, _:natural-numbers) : =>multisets(GT)
/* `multiset-delete(MS, GV, N)` removes `N` copies of `V` from the multiset `MS`,
or all copies of `GV` if there are fewer than `N` in `MS`.
*/ Built-in Funcon is-submultiset(_:multisets(GT), _:multisets(GT)) : =>booleans
/* `is-submultiset(MS1, MS2)` tests whether every element of `MS1` has equal or
fewer occurrences in `MS1` than in `MS2`.
*/
/*`multisets(GT)` is the type of possibly-empty finite multisets of elements of `GT`.