(2017a) counts the incorrect labels (i, j, X) in the predicted tree: Note that X can be null ∅, and t * (i,j) denotes the gold label for span (i, j), which could also be ∅.
6 there are two cases where t * (i,j) = ∅: a subspan (i, j) due to binarization (e.g., a span combining the first two subtrees in a ternary branching node), or an invalid span in t that crosses a gold span in t * .