code: public Set<N> getChildren(N n)
      {
          Pair<HashSet<N>, HashSet<N>> p = nodes.get(n);
          
          if(p == null)
              return null;
          
          return p.getOutgoing();
      }
docs: Returns the set of all children of the requested node, or null if the node does not exist in the graph.
  @param n the node to obtain the children of
  @return the set of parents, or null if the node is not in the graph