code: public SharedTreeNode makeRootNode() {
      assert nodesArray.size() == 0;
      SharedTreeNode n = new SharedTreeNode(0, null, subgraphNumber, 0);
      n.setInclusiveNa(true);
      nodesArray.add(n);
      rootNode = n;
      return n;
    }
docs: Make the root node in the tree.
  @return The node