langchain/output_parsers/combining.py/CombiningOutputParser/get_format_instructions
class CombiningOutputParser:    def get_format_instructions(self) -> str:
        """Instructions on how the LLM output should be formatted."""

        initial = f"For your first output: {self.parsers[0].get_format_instructions()}"
        subsequent = "\n".join(
            [
                f"Complete that output fully. Then produce another output, separated by two newline characters: {p.get_format_instructions()}"  # noqa: E501
                for p in self.parsers[1:]
            ]
        )
        return f"{initial}\n{subsequent}"