public function testTrackedFilesEmpty()
{
$t1 = new TrackedFiles([$this->directory_a]);
$t2 = new TrackedFiles([$this->directory_b]);
self::assertFalse($t1->modifiedAfter($t2));
self::assertFalse($t2->modifiedAfter($t1));
self::assertFalse($t1->modifiedAfter($t1));
self::assertFalse($t2->modifiedAfter($t2));
}