Bridging Textual Ellipses

Bridging Textual Ellipses Udo Hahn Michael Strube Katja Markert Freiburg University (~ Computational Linguistics Lab Europaplatz 1, D-79085 Freiburg, Germany {hahn, struhe, markert}@coling.uni-freiburg.de Abstract (1) a. We present a hybrid text understanding methodology for the resolution of textual ellipsis. It integrates language-independent conceptual criteria and language-dependent functional constraints. The methodologi- cal framework for text ellipsis resolution is the centering model that has been adapted to constraints reflecting the functional infor- mation structure within utterances, i.e., the distinction between context-bound and un- bound discourse elements. b. e. Der 316127" wird mit einem Nicke l-Metatt-llydride- Akku bestfickt. 0"he 316LT is - with a nickel-metal-hydride accu- mulator - equipped.) Der Rechner wird durch diesen neuartigen Akku ftir 4 Stunden mit Strom versorgt. (l'he computer is - because of this new type of ac- cumulator - for 4 hours - with power - provided.) Dariaberhinmls ist die Ladezeit mit 1,5 Smnden sehr kurz. (Also - is - the charge time of 1.5 hours quite short.) 1 Introduction Text phenomena, e.g., textual forms of ellipsis and anaphora are a challenging issue for the design of parsers for text understanding systems, since lack- ing recognition lacilities either result in referentially incoherent or invalid text knowledge representations. At the conceptual level, textual ellipsis (also called functional anaphora) relates a quasi-anaphoric expres- sion to its extrasentential antecedent by conceptual attributes (or roles) associated with that antecedent (see, e.g., the relation between "Ladezeit" (charge time) and "Al&u" (accumulator) in (lc) and (lb)). Thus, it complements the phenomenon of nominal anaphora, where an anaphoric expression is related to its antecedent in terms of conceptual generalization (as, e.g., "'Rechner" (computer) refers to "'316LT", a particular notebook, in (lb) and (la)). The resolu- tion of text-level nominal (and pronominal) anaphora contributes to the construction of referentially valid text knowledge bases, while the resolution of textual ellipsis yields referentially coherent text knowledge bases. Both phenomena tend to interact, as evidenced by the examples below. "Akku" (accumulator) in (lb) is a nominal anaphor referring to "Nickel-Metall- llydride-Akku" (nickel-metal-hydride accumulator) in (1 a), which, when resolved, provides the proper refer- ent for relating "Ladezeit" (charge time) in (lc) to it. In the case of textual ellipsis, the missing conceptual link between two discourse elements occurring in ad- jacent utterances must be inferred in order to establish the local coherence of the discourse (for an early state- merit of that idea, cf. Clark (1975)). In sentence (lc) the information is missing that "Ladezeit" (charge time) links up with "Akku" (accumulator). This re- lation can only be made explicit if conceptual knowl- edge about the domain, viz. the relation charge-time-of between the concepts CIIARGE-TIMF, and ACCUMU- LATOR, is available. The solution we propose is embedded within the centering model (Grosz et al., 1995). In this approach, discourse entities linking successive utterances ~in a discourse segment are organized in terms of centers. Local coherence in discourse is established whenever a center element of the previous utterance is associ- ated with an expression that has a valid semantic in- …

Content truncated to 500 words