code: def controlled_by(self, *control_qubits: Qid) -> 'Operation':
          
          # Avoids circular import.
          from cirq.ops import ControlledOperation
          if control_qubits is None or len(control_qubits) is 0:
              raise ValueError(
                  "Can't get controlled operation without control qubit. Op: {}"
                  .format(repr(self)))
          else:
              return ControlledOperation(control_qubits, self)
docs: Returns a controlled version of this operation.
  
          Args:
              control_qubits: Qubits to control the operation by. Required.