Second, to capture the dependencies on s <t , we explicitly model the dependencies among local latent variables by inputting z s t−1 to the sentence decoder, so that z s t is conditioned on z s <t and is expected to model smooth transitions in a long text.
generally speaking, to write a long text, a human writer first arranges contents and discourse structure (i.e., high-level planning) and then realizes the surface form of each individual part (low-level realization).