Parsing Ambiguous Structures using Controlled Disjunctions and Unary Quasi-Trees

Parsing Ambiguous Structures using Controlled Disjunctions and Unary Quasi-Trees Philippe Blache LPL - CNRS 29 Avenue Robert Schuman F-13621 Aix-en-Provence pb~ipl, univ-aix, fr Abstract The problem of parsing ambiguous structures concerns (i) their representation and (ii) the spec- ification of mechanisms allowing to delay and control their evaluation. We first propose to use a particular kind of disjunctions called controlled disjunctions: these formulae allows the represen- tation and the implementation of specific con- straints that can occur between ambiguous val- ues. But an efficient control of ambiguous struc- tures also has to take into account lexical as well as syntactic information concerning this object. We then propose the use of unapt quasi-trees specifying constraints at these different levels. The two devices allow an effÉcient implementa- tion of the control of the ambiguity. Moreover, they are independent from a particular formalism and can be used whatever the linguistic theory. 1 Introduction Most of the approaches dealing with ambi- guity are disambiguating techniques. This preliminary constatation seems trivial and relies on a simple presuposition: the am- biguous structures need to be disambiguated. However, this is not true from several reo spects. Machine translation is a good ex- ample: the ambiguity of a sentence in the source language needs very often to be pre- served and translated into the target one (eft (Wedekind97)). Another remark, in the same perspective: most of the disambiguating techniques rely on a single linguistic level. In other words, they generally make nse of lexical or syn- tactic or semantic information, exclusively. But a natural processing of natural language should not work in this way. All the linguis- tic levels of NLP (i.e. phonetic, phonologic, lexical, syntactic, semantic and pragmatic) have to be taken into account at the same time. In other words, processing ambigu- ity would have to be parallel, not sequen- tial. The problem is then to use ambiguous structures during the parse without blocking the analysis. In a first approximation, such a problem comes to parse using underspecified structures. We will see that this constitutes a part of the solution. The third and last preliminary remark fo- cuses on the control strategies for the evalu- ation of ambiguous structures. These strate- gies can rely on the formal properties of the ambiguous structure (for example the sim- plification of a disjunctive formula), on the contextual relations, etc. But the ambiguous objects can themselves bear important infor- mation specifying some restrictions. We will develop in this paper several examples illus- trating this point. The approach described here make an intensive use of this kind of cmmtraints, also called control relations. We present in this paper a technique called controlled disjunctions allowing to represent and implement an efficient control of am- bigmous structures at the lexical and phrase- structure level. We illustrate this technique using the HPSC framework, but it could be used in all kind of feature-based representa- tions. This approach relies (i) on the rep- resentation of constraints relations between the feature values and (ii) on the …

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