code: public String toOneLineString() {
      StringBuilder buf = new StringBuilder();
      if (children == null || children.length == 0) {
        buf.append(label);
      } else {
        buf.append('(').append(label);
        for (TreeGraphNode child : children) {
          buf.append(' ').append(child.toOneLineString());
        }
        buf.append(')');
      }
      return buf.toString();
    }
docs: Returns a <code>String</code> representation of this node and
  its subtree as a one-line parenthesized list.
  
  @return <code>String</code> representation of this subtree