docs: public SortedMap subMap(Object startKey, Object endKey) {
          if (comparator == null) {
              if (((Comparable) startKey).compareTo(endKey) <= 0)
                  return makeSubMap(startKey, endKey);
          } else {
              if (comparator.compare(startKey, endKey) <= 0)
                  return makeSubMap(startKey, endKey);
          }
          throw new IllegalArgumentException();
      }