async def make_query_engine(self, ctx: Context, ev: LoadNodeEvent) -> StopEvent:
        """
        Query engine construction step.

        Args:
            ctx (Context): context
            ev (LoadNodeEvent): event

        Returns:
            StopEvent: stop event

        """
        # make retriever and query engine
        retriever = LongRAGRetriever(
            grouped_nodes=ev.grouped_nodes,
            small_toks=ev.small_nodes,
            similarity_top_k=ev.similarity_top_k,
            vector_store=ev.index.vector_store,
        )
        query_eng = RetrieverQueryEngine.from_args(retriever, ev.llm)
        ctx.data["query_eng"] = query_eng

        return StopEvent(
            result={
                "retriever": retriever,
                "query_engine": query_eng,
                "index": ev.index,
            }
        )

        Query engine construction step.

        Args:
            ctx (Context): context
            ev (LoadNodeEvent): event

        Returns:
            StopEvent: stop event

        
llama-index-packs/llama-index-packs-longrag/llama_index/packs/longrag/base.py