static <A, B> Map<A, Set<B>> pairsToMapWithSetValues(LPair<A, B> l) {
  new Map<A, Set<B>> mm;
  for (Pair<A, B> p : unnull(l)) {
    Set<B> set = mm.get(p.a);
    if (set == null) mm.put(p.a, set = new HashSet);
    set.add(p.b);
  }
  ret mm;
}

Began life as a copy of #1020567