}

  private static <K, V> ImmutableSortedMap<K, V> copyOfInternal(
      Map<? extends K, ? extends V> map, Comparator<? super K> comparator) {

    if (map instanceof ImmutableSortedMap) {
      // TODO: Prove that this cast is safe, even though
      // Collections.unmodifiableSortedMap requires the same key type.
      @SuppressWarnings("unchecked")
      ImmutableSortedMap<K, V> kvMap = (ImmutableSortedMap<K, V>) map;