Many existing paraphrase models introduce latent variables to describe the derivation of c from x, e.g., with transformations (Heilman and Smith, 2010;Stern and Dagan, 2011) or alignments (Haghighi et al., 2005;Das and Smith, 2009;Chang et al., 2010).
we opt for a simpler paraphrase model without latent variables in the interest of efficiency.