public Char2IntSortedMap subMap(final Character from, final Character to) {
   return new UnmodifiableSortedMap(sortedMap.subMap(from, to));
 }
 public Char2IntSortedMap subMap(final Character from, final Character to) {
   return new SynchronizedSortedMap(sortedMap.subMap(from, to), sync);
 }