public void TestIsProperSupersetOf()
{
var a = new RedBlackSetTester<int>();
Assert.That(() => a.IsProperSupersetOf(null), Throws.InstanceOf<ArgumentNullException>());
Assert.That(a.IsProperSupersetOf(Enumerable.Empty<int>()), Is.False);
Assert.That(a.IsProperSupersetOf(Enumerable.Range(1, 1)), Is.False);
a.Add(2);
Assert.That(a.IsProperSupersetOf(Enumerable.Empty<int>()));
a.AddRange(_odd);
Assert.That(a.IsProperSupersetOf(_primes));
}