In the unsupervised setting, a variety of successful systems have leveraged lexical cohesion (Halliday and Hasan, 1976) -the idea that topically-coherent segments display consistent lexical distributions (Hearst, 1994;Utiyama and Isahara, 2001; Eisenstein and Barzilay, 2008).
such systems almost invariably focus on linear segmentation, while it is widely believed that discourse displays a hierarchical structure (Grosz and Sidner, 1986).