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

        :return: a LinearOperator that calls apply().

def operator(self):
        """Get a ``LinearOperator`` corresponding to apply().

        :return: a LinearOperator that calls apply().
        """
        # is projection the zero operator?
        if self.V.shape[1] == 0:
            N = self.V.shape[0]
            return ZeroLinearOperator((N, N))
        return self._get_operator(self.apply, self.apply_adj)