langchain/chains/qa_with_sources/retrieval.py/RetrievalQAWithSourcesChain/_reduce_tokens_below_limit
class RetrievalQAWithSourcesChain:    def _reduce_tokens_below_limit(self, docs: List[Document]) -> List[Document]:
        num_docs = len(docs)

        if self.reduce_k_below_max_tokens and isinstance(
            self.combine_documents_chain, StuffDocumentsChain
        ):
            tokens = [
                self.combine_documents_chain.llm_chain.llm.get_num_tokens(
                    doc.page_content
                )
                for doc in docs
            ]
            token_count = sum(tokens[:num_docs])
            while token_count > self.max_tokens_limit:
                num_docs -= 1
                token_count -= tokens[num_docs]

        return docs[:num_docs]