code: private static ImmutableMap<IRI, VocabularyEntry> extractVocabulary(@Nonnull Mapping mapping) {
  		return mapping.getRDFAtomPredicates().stream()
  				.filter(p -> p instanceof TriplePredicate)
  				.map(p -> (TriplePredicate) p)
  				.findFirst()
  				.map(p -> extractTripleVocabulary(mapping, p)
  						.collect(ImmutableCollectors.toMap(e -> e.name, e -> e)))
  				.orElseGet(ImmutableMap::of);
      }
docs: TODO: refactor so as to work with quads