def add_qubits(self: Self, qubits: Sequence[cirq.Qid]) -> Self:
        """Add `qubits` in the `|0>` state to a new state space and take the kron product.

        Args:
            qubits: Sequence of qubits to be added.

        Returns:
            NotImplemented: If the subclass does not implement this method.
            Self: A `cirq.SimulationState` with qubits added or `self` if there are no qubits to
                add.
        """
        if not qubits:
            return self
        return NotImplemented
Add `qubits` in the `|0>` state to a new state space and take the kron product.

        Args:
            qubits: Sequence of qubits to be added.

        Returns:
            NotImplemented: If the subclass does not implement this method.
            Self: A `cirq.SimulationState` with qubits added or `self` if there are no qubits to
                add.
        
cirq-core/cirq/sim/simulation_state.py