Sokil\Mongo\Pipeline::addStage PHP Method

    private function addStage($operator, $stage)
    {
        $lastIndex = count($this->stages) - 1;
        if ($operator == '$match' && isset($this->stages[$lastIndex][$operator])) {
            $this->stages[$lastIndex][$operator] = array_merge($this->stages[$lastIndex][$operator], $stage);
        } else {
            $this->stages[] = array($operator => $stage);
        }
    }