code: @Override
      public Set<URI> listModelReferences(URI elementUri) {
          if (elementUri == null) {
              return ImmutableSet.of();
          }
  
          return ImmutableSet.copyOf(this.modelReferencesMap.get(elementUri));
      }
docs: Obtains the list of model references for a given element.
  
  @param elementUri the URI of the element for which we want to obtain the model references
  @return a List of URIs with the model references of the given element. If there are no model references the List should be empty NOT null.