static MultiSet collectMultiSet(Collection c, S field) {
  new MultiSet set;
  for (O a : c) {
    O val = getOpt(a, field);
    if (val != null)
      set.add(val);
  }
  ret set;
}

static MultiSet mapMethodLike collectMultiSet(S field, Collection c) {
  ret collectMultiSet(c, field);
}

Began life as a copy of #1002504