For efficiency, we approximate this using the onebest predicted tree from a pre-trained Berkeley parser, 8 denoted asŷ berk (x * ), as follows: whereŷ berk <j (x * ) are all the non-terminals in y berk (x * ) that occur before x * j .
we first parse the sentence with a Berkeley parser, and use the resulting tree prefix as conditioning context to compute the probability of generating w ∈ Σ under the RNNG.