code: public Collection<VM> getAssociatedVGroup(VM u) {
          for (Collection<VM> vGrp : sets) {
              if (vGrp.contains(u)) {
                  return vGrp;
              }
          }
          return Collections.emptySet();
      }
docs: Get the group of VMs that contains the given VM.
  
  @param u the VM identifier
  @return the group of VM if exists, an empty collection otherwise