Repair Strategies for Lexicalized Tree Grammars

Proceedings of EACL '99 Repair Strategies for Lexicalized Tree Grammars Patrice Lopez LORIA, BP239, 54500 Vandoeuvre, FRANCE lopez@loria.fr Abstract This paper presents a framework for the definition of monotonic repair rules on chart items and Lexicalized Tree Gram- mars. We exploit island representations and a new level of granularity for the linearization of a tree called connected routes. It allows to take into account the topology of the tree in order to trigger additional rules. These local rules cover ellipsis and common extra-grammatical phenomena such as self-repairs. First re- sults with a spoken language corpora are presented. Introduction In the context of spoken task-oriented man- machine and question-answering dialogues, one of the most important problem is to deal with spon- taneous and unexpected syntactical phenomena. Utterances can be very incomplete and difficult to predict which questions the principle of gram- maticality. Moreover large covering grammars are generally dedicated to written text parsing and it is not easy to exploit such a grammar for the analysis of spoken language even if complex syn- tax does not occur. For such sentences, robust parsing techniques are necessary to extract a maximum of informa- tion from the utterance even if a Complete parsing fails (at least all possible constituents). Consid- ering parsing of word-graphs and the large search space of parsing algorithms in order to compute all possible ambiguities, the number of partial parses can be very important. A robust semantic pro- cessing on these partial derivations would result in a prohibitive number of hypotheses. We argue in this paper that appropriate syntactical constraints expressed in a Lexicalized Tree Grammar (LTG) can trigger efficient repair rules for specific oral phenomena. First results of a classical grammatical parsing are presented, they show that robust parsing need to cope with oral phenomena. We argue then that extended domain of locality and lexicalization of LTG can be exploited in order to express repair local rules for these specific spoken phenomena. First results of this approach are presented. 1 LTG parsing and repairing strategy 1.1 Experimental results Table 1 presents parsing test results of the Go- cad corpora. This corpora contains 861 utterances in French of transcribed spontaneous spoken lan- guage collected with a Wizard of Oz experiment (Chapelier et al., 1995). We used a bottom-up parser (Lopez, 1998b) for LTAG. The size of the grammar was limited compared with (Candito, 1999) and corresponds to the sublanguage used in the Gocad application. However designing princi- ples of the grammar was close to the large covering French LTAG grammar just including additional elementary trees (for example for unexpected ad- verbs which can modify predicative nouns) and a notation enrichment for the possible ellipsis occur- rences (Lopez, 1998a). The LTAG grammar for the sublanguage corresponds to a syntactical lex- icon of 529 entries and a set of 80 non-instancied elementary trees. A taxonomy of parsing errors occurring in oral dialogue shows that the majority of failures are linked to orality: hesitations, repetitions, self re- pairs and some head ellipsis. The …

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