Implicit nonsmooth block in alternating proximal-gradient iteration

For $f(x,y)=h(x,y)+\varphi (y)$, where $h$ is convex with an $L_h$-Lipschitz gradient and $\varphi$ is convex, use a proximal $y$-step for the full section and an explicit $x$-step for $h$. Monotonicity of the two selected subgradients of $\varphi$ combines with cocoercivity of $\nabla h$ to make the full update firmly nonexpansive for $0<\tau L_h\le 1$. Existence of a minimizer then gives convergence, even though the full objective is not smooth.