(Koo et al., 2010;Anzaroot et al., 2014;Vilnis et al., 2015), many based on dual decomposition (Rush and Collins, 2012).
the constraints are often fixed, and even when learned (Anzaroot et al., 2014;Vilnis et al., 2015), the learning is done simply on constraint weights generated from pre-made templates, the construction of which requires domain knowledge.