if (subRange.equals(Range.all())) {
      return this;
    } else {
      return new SubRangeMap(subRange);
    }
  }

  @SuppressWarnings("unchecked")
  private RangeMap<K, V> emptySubRangeMap() {
    return (RangeMap<K, V>) (RangeMap<?, ?>) EMPTY_SUB_RANGE_MAP;
  }

  @SuppressWarnings("ConstantCaseForConstants") // This RangeMap is immutable.