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.
the grammar rule 2 → 4 1 already encodes the notion of the sentiment of the right child being dominant, so when this is conjoined with our span feature on the first word (While), we end up with a feature that captures this effect.