code: public void addDependency(Vertex<T> v)
      {
          if (!m_dependencies.contains(v))
          {
              m_dependencies.add(v);
          }
      }
docs: Add a dependecy to this Vertex.  The Vertex that this one depends on will
  be marked as referenced and then added to the list of dependencies.  The
  list is checked before the dependency is added.
  
  @param v  The vertex we depend on.