static S ai_parseTreeGrouping(PTElement e) {
  if (e == null) null;
  ChoosePart comp = ai_parseTree_descendToComposite(e);
  if (comp != null)
    ret curlyOpt(ai_parseTreeGrouping(first(comp.children))) + " "
      + curlyOpt(ai_parseTreeGrouping(second(comp.children)));
  ret e.text();
}