{New(elements2...), New(elements...)},
	} {
		s1, s2 := sets[0], sets[1]
		if got := s1.Union(s2); !got.Equals(want) {
			t.Errorf("expected %v; got %v", want, got)
		}
	}
}

func TestDifference(t *testing.T) {
	s1 := New("a", "b", "c", "d")
	s2 := New("a", "b", "e")
	want := New("c", "d")

	t.Run("difference", func(t *testing.T) {
		d := s1.Difference(s2)
		if !want.Equals(d) {