UnorderedMultiSet.swift

private func consistsOfSameElements<Element: Equatable>(between lhs: [Element], and rhs: [Element]) -> Bool {
    let contained = lhs.compactMap { element in
        return rhs.contains(element) ? element : nil
    }
    return contained == lhs
}

private func contains<Element: Equatable>(_ element: [Element], in collection: [[Element]]) -> Bool {
    return collection.contains { consistsOfSameElements(between: element, and: $0) }
}