Improving the running time of the Topological Ordering algorithm
by a constant factor.

In class Graph:

Node succ; Node n;
succ.predecessors.add(n)
succ.predecessors.remove(n)
succ.predecessors.size() == 0
n.predecessors.clear();


The Law of Demeter
says a class should interact directly with its collaborators and be 
shielded from understanding their internal structure.

When we only take our direct dependencies, we minimize coupling and maximize reusability.

We are interested in predCount and it is not a good idea to 
expose the details about predecessors.

succ.predCount++
succ.predCount--
succ.predCount == 0
n.predCount = 0

You shouldn't 
have to care that Server contains EmailSystem, which contains Sender.
You shouldn't
have to care Node contains an ArrayList of nodes.

See also:
~/.www/courses/se-courses/cs5500/f11/LoD