code: public void stem(CoreLabel label,
                     Class<? extends CoreAnnotation<String>> ann) {
      String lemma = lemmatize(label.word(), label.tag(), lexer, lexer.option(1));
      label.set(ann, lemma);
    }
docs: Adds annotation <code>ann</code> to the given CoreLabel.
  Assumes that it has a TextAnnotation and PartOfSpeechAnnotation.