static PTElement ai_parseBottomUpToTree(S input) {
  final new AI_BottomUpParser1 parser;
  parser.parse(input);
  AI_BottomUpParser1.Word w = parser.fullGroup();
  if (w == null) null;
  L<S> classes = withoutQuoted(w.classes);
  if (empty(classes)) null;
  ret ai_completeParseTree(parser, ChooseCategory(w, classes));
}