docs: public String toPrettyString(int indentLevel) {
      StringBuilder buf = new StringBuilder("\n");
      for (int i = 0; i < indentLevel; i++) {
        buf.append("  ");
      }
      if (children == null || children.length == 0) {
        buf.append(label.toString("value-index{map}"));
      } else {
        buf.append('(').append(label.toString("value-index{map}"));
        for (TreeGraphNode child : children) {
          buf.append(' ').append(child.toPrettyString(indentLevel + 1));
        }
        buf.append(')');
      }
      return buf.toString();
    }