static Set<Set<EKey>> join(Set<? extends Set<EKey>> sop1, Set<? extends Set<EKey>> sop2) {
    Set<Set<EKey>> sop = new HashSet<>();
    sop.addAll(sop1);
    sop.addAll(sop2);
    return sop;
  }
static Set<Set<EKey>> join(Set<? extends Set<EKey>> sop1, Set<? extends Set<EKey>> sop2)