static <A> MultiSet<A> multiSetWithoutSingles(MultiSet<A> ms) {
  if (ms == null) null;
  MultiSet<A> ms2 = similarEmptyMultiSet(ms);
  for (A a : keys(ms)) {
    int n = ms.get(a);
    if (n > 1)
      ms2.add(a, n);
  }
  ret ms2;
}