/**
  * Asynchronously explains why {@code instance} is an instance of {@code cl}
  *
  * @param instance The instance
  * @param cl The class
  * @param callback The {@link Callback} to execute after the explanation is done
  */
 public void instance(Resource instance, Resource cl, Callback<Graph> callback) {
   String query = selectQuery(instance.toString(), RDF.TYPE.stringValue(), cl.stringValue());
   query(query, callback);
 }
 /**
  * Explains why {@code instance} is an instance of {@code cl}
  *
  * @param instance The instance
  * @param cl The class
  * @return The explanation {@link org.openrdf.model.Graph Graph}
  * @throws PelletClientException If there is an error or the {@code instance} is not an instance
  *     of {@code cl}
  */
 public Graph instance(Resource instance, Resource cl) throws PelletClientException {
   String query = selectQuery(instance.toString(), RDF.TYPE.stringValue(), cl.stringValue());
   return query(query);
 }