langchain/chains/sequential.py/SequentialChain/_acall
class SequentialChain:    async def _acall(
        self,
        inputs: Dict[str, Any],
        run_manager: Optional[AsyncCallbackManagerForChainRun] = None,
    ) -> Dict[str, Any]:
        known_values = inputs.copy()
        _run_manager = run_manager or AsyncCallbackManagerForChainRun.get_noop_manager()
        callbacks = _run_manager.get_child()
        for i, chain in enumerate(self.chains):
            outputs = await chain.acall(
                known_values, return_only_outputs=True, callbacks=callbacks
            )
            known_values.update(outputs)
        return {k: known_values[k] for k in self.output_variables}