code: def qubits(self) -> Qubits:
          
          qbs = [q for elem in self.elements for q in elem.qubits]    # gather
          qbs = list(set(qbs))                                        # unique
          qbs = sorted(qbs)                                           # sort
          return tuple(qbs)
docs: Returns: Sorted list of qubits acted upon by this circuit
  
          Raises:
              TypeError: If qubits cannot be sorted into unique order.