docs: public void insert(NumberVector nv) {
      final int dim = nv.getDimensionality();
      // No root created yet:
      if(root == null) {
        ClusteringFeature leaf = new ClusteringFeature(dim);
        leaf.addToStatistics(nv);
        root = new TreeNode(dim, capacity);
        root.children[0] = leaf;
        root.addToStatistics(nv);
        ++leaves;
        return;
      }
      TreeNode other = insert(root, nv);
      // Handle root overflow:
      if(other != null) {
        TreeNode newnode = new TreeNode(dim, capacity);
        newnode.addToStatistics(newnode.children[0] = root);
        newnode.addToStatistics(newnode.children[1] = other);
        root = newnode;
      }
    }