langchain/output_parsers/rail_parser.py/GuardrailsOutputParser/from_rail_string
class GuardrailsOutputParser:    def from_rail_string(
        cls, rail_str: str, num_reasks: int = 1
    ) -> GuardrailsOutputParser:
        try:
            from guardrails import Guard
        except ImportError:
            raise ValueError(
                "guardrails-ai package not installed. "
                "Install it by running `pip install guardrails-ai`."
            )
        return cls(guard=Guard.from_rail_string(rail_str, num_reasks=num_reasks))