def ystep(self):
        r"""Minimise Augmented Lagrangian with respect to
        :math:`\mathbf{y}`.
        """

        self.Y = np.asarray(sp.prox_l2(
            self.AX + self.U, (self.lmbda/self.rho)*self.Wtvna,
            axis=self.saxes), dtype=self.dtype)
r"""Minimise Augmented Lagrangian with respect to
        :math:`\mathbf{y}`.