} else {
          return result;
        }
      }
    };
  }

  /** Returns the first node in the tree that is in range. */
  @CheckForNull
  private AvlNode<E> firstNode() {
    AvlNode<E> root = rootReference.get();
    if (root == null) {
      return null;
    }
    AvlNode<E> node;
    if (range.hasLowerBound()) {