private @Nullable AvlNode<E> firstNode() {
    AvlNode<E> root = rootReference.get();
    if (root == null) {
      return null;
    }
    AvlNode<E> node;
    if (range.hasLowerBound()) {
      // The cast is safe because of the hasLowerBound check.
      E endpoint = uncheckedCastNullableTToT(range.getLowerEndpoint());
      node = root.ceiling(comparator(), endpoint);
      if (node == null) {
        return null;
      }

  private @Nullable AvlNode<E> firstNode() {
    AvlNode<E> root = rootReference.get();
    if (root == null) {
      return null;
    }
    AvlNode<E> node;
    if (range.hasLowerBound()) {
      // The cast is safe because of the hasLowerBound check.
      E endpoint = uncheckedCastNullableTToT(range.getLowerEndpoint());
      node = root.ceiling(comparator(), endpoint);
      if (node == null) {
        return null;
      }

        && map.comparator() != Ordering.natural()
        && range.hasLowerBound()
        && range.hasUpperBound()) {
      checkArgument(
          map.comparator().compare(range.lowerEndpoint(), range.upperEndpoint()) <= 0,
          "map is using a custom comparator which is inconsistent with the natural ordering.");
    }
    if (range.hasLowerBound() && range.hasUpperBound()) {
      return map.subMap(