public void TestExceptWith()
{
var a = new RedBlackSetTester<int>(_numbers);
Assert.That(() => a.ExceptWith(null), Throws.InstanceOf<ArgumentNullException>());
a.ExceptWith(_odd);
Assert.That(a, Is.EqualTo(_even.Where(x => x <= _numbers.Max())));
a.ExceptWith(_primes);
Assert.That(a, Is.EqualTo(_even.Where(x => x > 2 && x <= _numbers.Max())));
}