public void testIntersection() {
List<int> intersection = SetOps.intersection(s1, s2);
Assert.AreEqual(1, intersection.Count);
Assert.AreEqual(4, s1.Count);
Assert.AreEqual(3, s2.Count);
s1.Remove(1);
Assert.AreEqual(1, intersection.Count);
Assert.AreEqual(3, s1.Count);
Assert.AreEqual(3, s2.Count);
}