static PTElement ai_parseTree_simplifyNode(PTElement e) {
  if (e == null) null;
  e = ai_parseTree_descendToComposite(e);
  if (e == null) null;
  if (e instanceof ChoosePart) {
    ai_Reconstructed r = e/ChoosePart.reconstruction;
    WeightedProduction prod = r.production;
    S hint = prod.simplificationHint;
    if (eq(hint, "L")) ret first(e.children);
    if (eq(hint, "R")) ret second(e.children);
  }
  ret e;
}