For non-terminal productions, we back off from r to P and its parent P , and then to just P .
we back off from a rule-annotated grammar p(C d 1 |P, P , r ) to a parent-annotated grammar (Johnson, 1998)  In order to generalize to unseen rule yields C d 1 , we further back off from the basic PCFG probability In other words, we back off from a raw PCFG to where λ = 0.9 is an interpolation constant.