docs: 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)