public function testPipeline_MatchCallable()
 {
     $pipeline = new Pipeline($this->collection);
     $pipeline->match(function ($expression) {
         /* @var $expression \Sokil\Mongo\Expression */
         $expression->where('a', 1)->whereLess('b', 12);
     });
     $this->assertEquals('[{"$match":{"a":1,"b":{"$lt":12}}}]', (string) $pipeline);
 }