static <A, B> MultiMap<A, B> mergeMapsToMultiMap(Map<A, B>... maps) {
  MultiMap<A, B> mm = null;
  for (Map<A, B> m : maps) {
    if (mm == null) mm = emptyMultiMapSimilarToMap(m);
    mm.putAll(m);
  }
  ret uniquifyMultiMapValues(mm);
}