public function testSub()
{
$d1 = new QtiDuration('PT2S');
$d2 = new QtiDuration('PT1S');
$d1->sub($d2);
$this->assertEquals('PT1S', $d1->__toString());
$d1 = new QtiDuration('PT2S');
$d2 = new QtiDuration('PT4S');
$d1->sub($d2);
$this->assertEquals('PT0S', $d1->__toString());
$d1 = new QtiDuration('P1DT2H25M30S');
$d2 = new QtiDuration('P1DT2H');
$d1->sub($d2);
$this->assertEquals('PT25M30S', $d1->__toString());
$d1 = new QtiDuration('PT20S');
$d2 = new QtiDuration('PT20S');
$d1->sub($d2);
$this->assertEquals('PT0S', $d1->__toString());
$d1 = new QtiDuration('PT20S');
$d2 = new QtiDuration('PT21S');
$d1->sub($d2);
$this->assertTrue($d1->isNegative());
}
