code: public Collection<Node> getAssociatedPGroup(Node u) {
          for (Collection<Node> pGrp : pGroups) {
              if (pGrp.contains(u)) {
                  return pGrp;
              }
          }
          return Collections.emptySet();
      }
docs: Get the group of nodes associated to a given node.
  
  @param u the node
  @return the associated group of nodes if exists. An empty set otherwise