public function testXKDiffAcceptsFunction()
 {
     $xao = new XArray(['x' => 1, 'y' => 2, 'z' => 3]);
     $obj = $xao->xkdiff(['ab' => 4, 'foo', 'bar'], 'length_compare_func');
     $this->assertEquals(['ab' => 4], (array) $obj);
 }