code: 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;
    }
docs: Gets the complete set of POS tags used for any word in
  {@code examples}.
  
  @param examples
  @return