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