def noisy_moments(
        self, moments: Iterable[cirq.Moment], system_qubits: Sequence[cirq.Qid]
    ) -> Sequence[cirq.OP_TREE]:
        """Adds possibly stateful noise to a series of moments.

        Args:
            moments: The moments to add noise to.
            system_qubits: A list of all qubits in the system.

        Returns:
            A sequence of OP_TREEs, with the k'th tree corresponding to the
            noisy operations for the k'th moment.
        """
        raise NotImplementedError
Adds possibly stateful noise to a series of moments.

        Args:
            moments: The moments to add noise to.
            system_qubits: A list of all qubits in the system.

        Returns:
            A sequence of OP_TREEs, with the k'th tree corresponding to the
            noisy operations for the k'th moment.
        
cirq-core/cirq/devices/noise_model.py