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

        AXU = self.AX + self.U
        self.block_sep0(self.Y)[:] = sp.prox_l1(
            self.block_sep0(AXU), (self.lmbda/self.rho) * self.Wl1)
        self.block_sep1(self.Y)[:] = sp.prox_l2(
            self.block_sep1(AXU), self.mu/self.rho, axis=(self.cri.axisC, -1))
r"""Minimise Augmented Lagrangian with respect to
        :math:`\mathbf{y}`.