docs: public void addExistingItem(final ItemsNodeInterval newNode) {
          Preconditions.checkState(newNode.getItems().size() == 1, "Invalid node=%s", newNode);
          final Item item = newNode.getItems().get(0);
  
          addNode(newNode,
                  new AddNodeCallback() {
                      @Override
                      public boolean onExistingNode(final NodeInterval existingNode) {
                          final ItemsInterval existingOrNewNodeItems = ((ItemsNodeInterval) existingNode).getItemsInterval();
                          existingOrNewNodeItems.add(item);
                          // There is no new node added but instead we just populated the list of items for the already existing node
                          return false;
                      }
  
                      @Override
                      public boolean shouldInsertNode(final NodeInterval insertionNode) {
                          // Always want to insert node in the tree when we find the right place.
                          return true;
                      }
                  });
      }