code: public static Vocab of(Vocab... vocabs)
    {
      return new AggregateVocab(new ImmutableList.Builder<Vocab>().add(vocabs).build());
    }
docs: Returns a vocabulary composed of the union of the vocabularies given as
  parameter. The given vocabularies must have the same base URI.
  
  @param vocabs
  the vocabularies to aggregate.
  @return the aggregated vocabulary.