static <A, B> void dropValuesFromMapOtherThan(Map<A, B> map, B... values) {
  HashSet<B> valueSet = new HashSet(Arrays.asList(values));
  for (Iterator<Map.Entry<A, B>> it = map.entrySet().iterator(); it.hasNext();)
    if (!valueSet.contains(it.next().getValue())) it.remove();
}