As shown in Rubinstein and Kroese (2008) , the optimal choice of the proposal distribution is in fact the true posterior p(a|x), in which case the importance weight p(a,x) p(a|x) = p(x) is constant with respect to a. In<cite> Dyer et al. (2016)</cite> , the proposal distribution depends on x, i.e., q(a) q(a|x), and is computed with a separately-trained, discriminative model.