public void AddSuccessorTest()
{
Node <City> ct = new Node <City> (new City("war", "maz", "bbb"));
Node <City> r = n.AddSuccessor(ct);
Assert.Contains(ct, n.successors);
Assert.Contains(ct, r.successors);
Assert.Contains(n, ct.predecessors);
Assert.AreSame(n, r);
}