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