Complex discourse units and their semantics

International audienceA natural and intuitive principle concerning the organization of content in discourse is that discourse structure and rhetorical function operate at several levels of granularity at once. There are low level discourse connections between elementary discourse units (EDUs), even within a single sentence; but there are also discourse connections between larger constituents, complex discourse units or CDUs, which may include only two or three EDUs or may correspond to several paragraphs. CDUs and the constraints they impose on the discourse structure have not been an object of study in computational or formal work on discourse, as they are generally the by-product of processes either focused on elementary units (eg in RST) or on thematic cohesion (eg in text tiling). The purpose of this paper is to fill this lacuna. First, we give some more details about the importance of CDUs in an account of discourse structure. We then provide formal definitions of equivalences involving discourse graphs, which enables us to prove some results about how CDUS relate to EDUs and to each other. This in turn leads us to provide separation axioms or existence principles for CDUs. We work within the framework of SDRT (Asher 1993, Asher and Lascarides 2003)