Generating referring expressions with a unification grammar

Proceedings of EACL '99 Generating referring expressions with a unification grammar Richard Power Information Technology Research Institute University of Brighton Lewes Road Brighton BN2 4AT, UK Richard.Power@itri.bton.ac.uk Abstract A simple formalism is proposed to repre- sent the contexts in which pronouns, def- inite/indefinite descriptions, and ordinal descriptions (e.g. 'the second book') can be used, and the way in which these ex- pressions change the context. It is shown that referring expressions can be gener- ated by a unification grammar provided that some phrase-structure rules are spe- cially tailored to express entities in the current knowledge base. 1 Introduction Nominal referring expressions are exceptionally sensitive to linguistic context. If a discourse men- tions a book, potential referring expressions in- clude 'it', 'a book', 'the book', 'another book', 'the second book', along with an unlimited num- ber of more complex descriptions (e.g. 'the red book') that mention the book's properties. The choice among these alternatives depends on fea- tures of the preceding text: whether the referent has been mentioned before; whether it is currently a focus of attention; whether different referents of the same type (e.g. other books) have been in- troduced as well. Taking account of such factors poses a tricky problem for Natural Language Gen- eration (NLG), especially in applications in which efficiency (i.e. fast generation of texts) is a prior- ity. This paper proposes a method that allows effi- cient generation of referring expressions, through a unification grammar, at the cost of some ini- tial effort in tailoring the phrase-structure rules to the current knowledge base. The method was invented to meet the needs of applications us- ing 'WYSIWYM editing' (Power and Scott, 1998), which allow an author to control the content of an automatically generated text without prior train- ing in knowledge engineering. WYSIWYM is based [ 1 oAL j 1 _f ] procedure j r ~ put-on j -[, patch METHOD ~ I REST I Figure 1: Network representation of an instruction on the idea of a 'feedback text', i.e. a text, gener- ated by the system, that presents the current con- tent of the knowledge base (however incomplete) along with the set of permitted operations for ex- tending or otherwise editing the knowledge; these operations are provided through pop-up menus which open on spans of the feedback text. Two re- quirements of WYSIWYM editing are that feedback texts should be generated fast (even a delay of a few seconds is irritating), and that they should ex- press coreference relations clearly through appro- priate referring expressions; reconciling these two requirements has motivated the work described here. The semantic network in figure 1 shows a knowl- edge base that might be produced using the ICON- OCLAST 1 system, which generates patient infor- mation leaflets. At present this knowledge base defines only the goal and first step of a procedure; before generating a useful output text the author would have to add further steps. To facilitate the author's task, the program generates the following feedback text, including the 'anchor' …

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