  Implicit discourse relation recognition is a challenging task that involves
identifying the sense or senses that hold between two adjacent spans of text,
in the absence of an explicit connective between them. In both PDTB-2 and
PDTB-3, discourse relational senses are organized into a three-level hierarchy
ranging from four broad top-level senses, to more specific senses below them.
Most previous work on implicit discourse relation recognition have used the
sense hierarchy simply to indicate what sense labels were available. Here we do
more -- incorporating the sense hierarchy into the recognition process itself
and using it to select the negative examples used in contrastive learning. With
no additional effort, the approach achieves state-of-the-art performance on the
task.
