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An object that is a has a classical control key or keys.
Control keys are used in referencing the results of a measurement.
Users should implement
_control_keys_ returning an iterable of
_control_keys_() -> Union[FrozenSet['cirq.MeasurementKey'], NotImplementedType, None]
Return the keys for controls referenced by the receiving object.
|The measurement keys the value is controlled by. If the value is not classically controlled, the result is the empty tuple.|