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; Attardi and Dell'Orletta, 2009) , and approaches that combine independently-trained models only at parsing time (Sagae and Lavie, 2006; <cite>Hall et al., 2007</cite>; Attardi and Dell'Orletta, 2009 ).