function testSubtract()
{
$_SESSION['test'] = 'MemModel subtract test';
$model1 = new MemModel();
$stat = new Statement(new Resource('http://www.example.org/sub1'), new Resource('http://www.example.org/pred1'), new Resource('http://www.example.org/obj1'));
$stat2 = new Statement(new Resource('http://www.example.org/sub2'), new Resource('http://www.example.org/pred2'), new Resource('http://www.example.org/obj2'));
$stat3 = new Statement(new Resource('http://www.example.org/sub3'), new Resource('http://www.example.org/pred3'), new Resource('http://www.example.org/obj3'));
$model1->add($stat);
$model1->add($stat2);
$model2 = new MemModel();
$model2->add($stat);
$model2->add($stat2);
$model2->add($stat3);
$res = new MemModel();
$res = $model2->subtract($model1);
$this->assertEqual(1, $res->size());
$this->assertTrue($res->contains($stat3));
}