In this work, we refine the one-category grammar introduced by Chiang (2007) for HPBT in order to learn additional latent NT categories.
the following discussion is restricted to these kinds of grammars, although the method is equally applicable in other scenarios, e.g., the extended treeto-string transducer (xRs) formalism (Huang et al., 2006;Graehl et al., 2008) commonly used in syntax-directed translation, and phrase-based MT (Koehn et al., 2003).