public function testXIntersectAcceptsCallback()
 {
     $xao = new XArray([1, 2, 'foo' => 'bar', 'x' => 'y', 4]);
     $test = new CallbackTestMethods();
     $obj = $xao->xintersect(['xxx', 18], [$test, 'length_compare']);
     $this->assertEquals(['xxx'], (array) $obj);
 }