While classifying explicit discourse relations is relatively easy, as the discourse connective itself provides a strong cue for the discourse relation (Pitler et al., 2008), the classification of implicit discourse relations has proved to be notoriously hard and remained one of the last missing pieces in an end-to-end discourse parser .
we focus on the second-level 11-way classification because the labels are fine-grained enough to be useful for downstream tasks and also because the strongest neural network systems are tuned to this formulation.