Get a ``LinearOperator`` corresponding to apply_complement().

        :return: a LinearOperator that calls apply_complement().

def operator_complement(self):
        """Get a ``LinearOperator`` corresponding to apply_complement().

        :return: a LinearOperator that calls apply_complement().
        """
        # is projection the zero operator? --> complement is identity
        if self.V.shape[1] == 0:
            N = self.V.shape[0]
            return IdentityLinearOperator((N, N))
        return self._get_operator(self.apply_complement,
                                  self.apply_complement_adj)