docs: public Tokenizer<Tree> getTokenizer(final Reader r) {
      return new AbstractTokenizer<Tree>() {
        TreeReader tr = trf.newTreeReader(r);
        @Override
        public Tree getNext() {
          try {
            return tr.readTree();
          }
          catch(IOException e) {
            System.err.println("Error in reading tree.");
            return null;
          }
        }
      };
    }