However, negative features-features that are not observed in any tree-are still powerful indicators of (un)grammaticality: if we have never seen a PRN that starts with "has," or a span that begins with a quotation mark and ends with a close bracket, then we would like the model to be able to place negative weights on these features.
we use a simple feature hashing scheme where positive features are indexed individually, while nega-  Our goal is to use surface features to replicate the functionality of other annotations, without increasing the state space of our grammar, meaning that the rules rule(r) remain simple, as does the state space used during inference.