langchain/chains/sequential.py/SimpleSequentialChain/validate_chains
class SimpleSequentialChain:    def validate_chains(cls, values: Dict) -> Dict:
        """Validate that chains are all single input/output."""
        for chain in values["chains"]:
            if len(chain.input_keys) != 1:
                raise ValueError(
                    "Chains used in SimplePipeline should all have one input, got "
                    f"{chain} with {len(chain.input_keys)} inputs."
                )
            if len(chain.output_keys) != 1:
                raise ValueError(
                    "Chains used in SimplePipeline should all have one output, got "
                    f"{chain} with {len(chain.output_keys)} outputs."
                )
        return values