There are several attempts at incorporating arbitrary tree-based features but these involve either solving an ILP problem (Riedel and Clarke, 2006) or using computa- tionally intensive sampling-based methods (Nakagawa, 2007).
these models, while accurate, are slow (O(n 3 ) for projective, first-order models, higher polynomials for higher-order models, and worse for richer tree-feature models).