docs: public static Set<String> getPosTagVocabulary(Iterable<? extends CcgExample> examples) {
      Set<String> posTagVocabulary = Sets.newHashSet();
      for (CcgExample example : examples) {
        posTagVocabulary.addAll(example.getSentence().getPosTags());
      }
      return posTagVocabulary;
    }