Such features are not suited to the problem of determining the difficulty of sentences in isolation, but they have also been shown to better predict readability for second-language learners compared to 'traditional' readability measures like those described above (Crossley et al., 2011).
the parser must now keep track of two states: (1) the fact that we are still looking for a VP to complete the overall sentence; and (2) the fact that we now have a relative clause to parse before we can complete the current NP.