public function group() : Stage\Group
    {
        $stage = new Stage\Group($this);
        $this->addStage($stage);

        return $stage;
    }
public function group() : Stage\Group
    {
        $stage = new Stage\Group($this);
        $this->addStage($stage);

        return $stage;
    }
public function group ( ) : Stage \ Group { $ stage = new Stage \ Group ( $ this ) ; $ this -> addStage ( $ stage ) ; return $ stage ; }
Groups documents by some specified expression and outputs to the next
stage a document for each distinct grouping.

@see http://docs.mongodb.org/manual/reference/operator/aggregation/group/
Groups documents by some specified expression and outputs to the next stage a document for each distinct grouping .