public function testKDiffAcceptsCallback()
 {
     $xao = new XArray(['x' => 1, 'ab' => 2, 'z' => 3]);
     $test = new CallbackTestMethods();
     $obj = $xao->kdiff(['x' => 4], [$test, 'length_compare']);
     $this->assertEquals(['ab' => 2], (array) $obj);
 }