static MultiMap<S, PairS> ai_pairsThatAreIdenticalWhenJoinedWithSpace(Collection<PairS> l) {
  MultiMap<S, PairS> mm = ciMultiMap();
  fOr (PairS p : l)
    mm.put(p.a + " " + p.b, p);
  ret keysWithMultipleValuesOnly(mm);
}