return func(m *BiMultimap) error {
		got, _ := m.Select(key)
		if !unorderedEqual(want, got) {
			return fmt.Errorf("forward select %v wanted %v. got %v.", key, want, got)
		}
		return nil
	}
}

func reverseSelect(key Key, want ...Key) expectation {
	return func(m *BiMultimap) error {
		got, _ := m.ReverseSelect(key)
		if !unorderedEqual(want, got) {