While it is easy to estimate the consequences of restrictions to n-grams of limited size, it is less trivial to estimate the consequences of the structural constraints imposed by syntax-based machine translation formalisms. Consequently, much work has been devoted to this task (Wu, 1997; Zens and Ney, 2003;<cite> Wellington et al., 2006</cite>; Macken, 2007; Søgaard and Kuhn, 2009) .