These approaches can generally be classified in two categories: models that integrate base parsers at learning time, e.g., using stacking (Nivre and McDonald, 2008;<cite> Attardi and Dell'Orletta, 2009)</cite> , and approaches that combine independently-trained models only at parsing time (Sagae and Lavie, 2006; Hall et al., 2007; <cite>Attardi and Dell'Orletta, 2009</cite> ).