public void TestIsProperSubsetOf()
{
var a = new RedBlackSetTester<int>();
Assert.That(() => a.IsProperSubsetOf(null), Throws.InstanceOf<ArgumentNullException>());
Assert.That(a.IsProperSubsetOf(Enumerable.Range(1, 1)));
Assert.That(a.IsProperSubsetOf(Enumerable.Empty<int>()), Is.False);
a.AddRange(_primes);
Assert.That(a.IsProperSubsetOf(_numbers));
a.AddRange(_even.Where(UpTo100));
Assert.That(a.IsProperSubsetOf(_numbers));
a.AddRange(_odd.Where(UpTo100));
Assert.That(a.IsProperSubsetOf(_numbers), Is.False);
}