docs: public List<Node> siblingNodes() {
          if (parentNode == null)
              return Collections.emptyList();
  
          List<Node> nodes = parentNode.ensureChildNodes();
          List<Node> siblings = new ArrayList<>(nodes.size() - 1);
          for (Node node: nodes)
              if (node != this)
                  siblings.add(node);
          return siblings;
      }