langchain/memory/summary.py/SummarizerMixin/predict_new_summary
class SummarizerMixin:    def predict_new_summary(
        self, messages: List[BaseMessage], existing_summary: str
    ) -> str:
        new_lines = get_buffer_string(
            messages,
            human_prefix=self.human_prefix,
            ai_prefix=self.ai_prefix,
        )

        chain = LLMChain(llm=self.llm, prompt=self.prompt)
        return chain.predict(summary=existing_summary, new_lines=new_lines)