docs: protected StringBuilder printDebug(StringBuilder buf, ClusteringFeature n, int d) {
      FormatUtil.appendSpace(buf, d).append(n.n);
      for(int i = 0; i < n.getDimensionality(); i++) {
        buf.append(' ').append(n.centroid(i));
      }
      buf.append(" - ").append(n.n).append('\n');
      if(n instanceof TreeNode) {
        ClusteringFeature[] children = ((TreeNode) n).children;
        for(int i = 0; i < children.length; i++) {
          ClusteringFeature c = children[i];
          if(c != null) {
            printDebug(buf, c, d + 1);
          }
        }
      }
      return buf;
    }