// preprocessor: func(S -> O)
// returns func(S -> E)
static O makeGrabber(OccTree2 tree, final O preprocessor) {
  final new StrictTreeMap<O, S> strictMap; // TODO: maybe use hashmap
  strictMap.detailedExceptions = true;
  for (OccTree2 node : tree.allNodes()) {
    if (node.e.a() && node.parent != null && node.parent.e.q()) {
      S in = node.parent.e.text(), out = node.e.text();
      O preprocessed;
      try {
        preprocessed = callFunction(preprocessor, in); // may throw exception, then we just skip
      } catch { continue; }
      
      if (preprocessed != null)
        strictMap.put(preprocessed, out);
    }
  }
  ret func(S in) {
    O preprocessed = callF(preprocessor, in);
    if (preprocessed == null) null;
    ret E.a(strictMap.get(preprocessed));
  };
}