public function testRemoveSubjectsOfType()
 {
     $graph = new ExtendedGraph();
     $graph->add_resource_triple("http://test/1", ExtendedGraph::rdf . 'type', self::ONT_resource . 'Item');
     $graph->add_resource_triple("http://test/2", ExtendedGraph::rdf . 'type', self::ONT_resource . 'Item');
     $graph->add_resource_triple("http://test/3", ExtendedGraph::rdf . 'type', self::ONT_resource . 'List');
     $graph->remove_subjects_of_type(self::ONT_resource . 'Item');
     $subjects = $graph->get_subjects();
     $this->assertEquals(1, count($subjects));
     $this->assertEquals("http://test/3", $subjects[0]);
 }