public function testRemoveProperties()
 {
     $graph = new ExtendedGraph();
     $graph->add_literal_triple('http://some/subject/1', 'http://some/predicate', 'some object');
     $graph->add_literal_triple('http://some/subject/2', 'http://some/predicate/to/remove', 'some object');
     $graph->add_resource_triple('http://some/subject/3', 'http://some/predicate/to/remove', 'http://some/other/object');
     $graph->add_literal_triple('http://some/subject/4', 'http://some/predicate', 'some object');
     $graph->remove_properties('http://some/predicate/to/remove');
     $this->assertFalse($graph->subject_has_property('http://some/subject/2', 'http://some/predicate/to/remove'), 'should have removed triple about subject 2');
     $this->assertFalse($graph->subject_has_property('http://some/subject/3', 'http://some/predicate/to/remove'), 'should have removed triple about subject 3');
 }