public function delete($docs)
    {
        if($docs instanceof Document) {
            $this->obj_gateway->delete([$docs->getId()]);
        } elseif (is_string($docs)) {
            $this->obj_gateway->delete([$docs]);
        } elseif (is_array($docs)) {
            $arr_doc_ids = [];
            foreach($docs as $doc){
                if($doc instanceof Document) {
                    $arr_doc_ids[] = $doc->getId();
                } elseif (is_string($doc)) {
                    $arr_doc_ids[] = $doc;
                } else {
                    throw new \InvalidArgumentException('Parameter must be one or more \Search\Document objects or ID strings');
                }
            }
            $this->obj_gateway->delete($arr_doc_ids);
        } else {
            throw new \InvalidArgumentException('Parameter must be one or more \Search\Document objects or ID strings');
        }
    }