Returns: Sorted list of qubits acted upon by this circuit

        Raises:
            TypeError: If qubits cannot be sorted into unique order.

def qubits(self) -> Qubits:
        """Returns: Sorted list of qubits acted upon by this circuit

        Raises:
            TypeError: If qubits cannot be sorted into unique order.
        """
        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)