Assuming that the semantic frame f of the predicate has already been identified (see §4.2 for this step), we seek to predict the set of spans that correspond to its overt semantic arguments and to label each argument with its semantic role.
we model the problem as that of assigning each span s ∈ S, from an over-generated set of candidate argument spans S, to a semantic role r ∈ R. The set of semantic roles R includes the special null role ∅, which is used to represent non-overt arguments.