def _measurement_key_objs_(self) -> frozenset[cirq.MeasurementKey] | NotImplementedType | None:
        """Return the key objects for measurements performed by the receiving object.

        When a measurement occurs, either on hardware, or in a simulation,
        these are the key values under which the results of the measurements
        will be stored.
        """
Return the key objects for measurements performed by the receiving object.

        When a measurement occurs, either on hardware, or in a simulation,
        these are the key values under which the results of the measurements
        will be stored.
        
cirq-core/cirq/protocols/measurement_key_protocol.py