def _control_keys_(self) -> frozenset[cirq.MeasurementKey] | NotImplementedType | None:
        """Return the keys for controls referenced by the receiving object.

        Returns:
            The measurement keys the value is controlled by. If the value is not
            classically controlled, the result is the empty tuple.
        """
Return the keys for controls referenced by the receiving object.

        Returns:
            The measurement keys the value is controlled by. If the value is not
            classically controlled, the result is the empty tuple.
        
cirq-core/cirq/protocols/control_key_protocol.py