public Reference2DoubleSortedMap<K> subMap(final K from, final K to) {
   return new UnmodifiableSortedMap<K>(sortedMap.subMap(from, to));
 }
 public Reference2DoubleSortedMap<K> subMap(final K from, final K to) {
   return new SynchronizedSortedMap<K>(sortedMap.subMap(from, to), sync);
 }