public function testGetDescendants()
{
$root = new Node('r');
$root->addChild($a = new Node('a'))->addChild($b = new Node('b'));
$a->addChild($c = new Node('c'))->addChild($d = new Node('d'));
$b->addChild($e = new Node('e'))->addChild($f = new Node('f'));
$this->assertEquals([$c, $d], $a->getDescendants());
$this->assertEquals([$e, $f], $b->getDescendants());
$this->assertEquals([$a, $c, $d, $b, $e, $f], $root->getDescendants());
}
