counter

type Counter[A any] struct {
	AddN           func(A, int) int
	RemoveN        func(A, int) int
	Get            func(A) int
	Len            func() int
	ToSlice        func() []*base.Pair[A, int]
	Union          func(*Counter[A]) *Counter[A]
	Intersect      func(*Counter[A]) *Counter[A]
	Difference     func(*Counter[A]) *Counter[A]
	Iterator       func() iterable.Iterator[*base.Pair[A, int]]
	KeysIterator   func() iterable.Iterator[A]
	CountsIterator func() iterable.Iterator[int]
}