Building off this work, a number of LS models using either words or other syntactic and semantic structures have been proposed for QA (Echihabi and Marcu, 2003;Soricut and Brill, 2006;Riezler et al., 2007;Surdeanu et al., 2011;Yao et al., 2013;Jansen et al., 2014), or related tasks, such as semantic textual similarity (Sultan et al., 2014).
all of these models fall into the class of first-order models.