code: def controlled_by(self, *control_qubits: Qid) -> 'Gate':
          
          # Avoids circular import.
          from cirq.ops import ControlledGate
          return ControlledGate(self, control_qubits,
                                len(control_qubits) if control_qubits is not None
                                                    else 1)
docs: Returns a controlled version of this gate.
  
          Args:
              control_qubits: Optional qubits to control the gate by.