code: @Override
      public Set<URI> listModelReferences(URI elementUri) {
          if (elementUri == null) {
              return ImmutableSet.of();
          }
  
          URI graphUri;
          try {
              graphUri = getGraphUriForElement(elementUri);
          } catch (URISyntaxException e) {
              log.warn("The namespace of the URI of the message content is incorrect.", e);
              return ImmutableSet.of();
          }
  
          if (graphUri == null) {
              log.warn("Could not obtain a graph URI for the element. The URI may not be managed by the server - " + elementUri);
              return ImmutableSet.of();
          }
  
          String queryStr = new StringBuilder()
                  .append("SELECT DISTINCT ?model WHERE { \n")
                  .append(" GRAPH <").append(graphUri.toASCIIString()).append("> { \n")
                  .append("<").append(elementUri.toASCIIString()).append("> <").append(SAWSDL.modelReference.getURI()).append("> ?model")
                  .append(" } \n ")
                  .append("} \n ")
                  .toString();
  
          return this.graphStoreManager.listResourcesByQuery(queryStr, "model");
      }
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 Set of URIs with the model references of the given element. If there are no model references the Set should be empty NOT null.