static void ai_printParseTree(PTElement e) {
  ai_printParseTree(e, "");
}

static void ai_printParseTree(PTElement e, S indent) {
  if (e == null) ret with print("No parse tree");
  print(indent + e);
  indent += "  ";
  for (PTElement child : unnull(e.children))
    ai_printParseTree(child, indent);
}