A head-dependents relation (HDR) is composed of a head and all its dependents, which can be viewed as an instance of a sentence pattern or phrase pattern.
since dependency trees are much flatter than constituency trees, the dependency-to-string model suffers more severe non-syntactic phrase coverage problem (Meng et al., 2013) than constituencybased models (Galley et al., 2004;Liu et al., 2006;Huang et al., 2006).