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Self{Monitoring with Reversible Grammars

Gunter Neumann? and Gertjan van Noord,y

?Deutsches Forschungszentrum

fur Kunstliche Intelligenz

Stuhlsatzenhausweg 3

D-6600 Saarbrucken 11

neumann@dfki.uni-sb.de

yRijksuniversiteit Groningen

Postbus 716

NL 9700 AS Groningen

vannoord@let.rug.nl

Abstract

We describe a method and its implementation for self-monitoring during natural language generation. In situations of communication where the generation of ambiguous utterances should be avoided our method is able to compute an unambiguous utterance for a given semantic input. The proposed method is based on a very strict integration of parsing and generation. During the monitored generation step, a previously generated (possibly) ambiguous utterance is parsed and the obtained alternative derivation trees are used as a `guide' for re-generating the utterance. To achieve such an integrated approach the underlying grammar must be reversible.

1 Introduction

In many situations of communication a speaker need not to worry about the possible ambiguity of what she is saying because she can assume that the hearer will be able to disambiguate the utterance by means of contextual information or would otherwise ask for clarification. But in some situations it is necessary to avoid the risk of generating ambiguous utterances that could lead to misunderstanding by the hearer, e.g., during the process of writing text, where no interaction is possible, or when utterances refer to actions that have to be performed directly or in some specific dialog situations (e.g. having an interview with a company).

The need to generate un-ambiguous utterances is also relevant for the development of natural language generation systems. For example in the case of an intelligent help-system that supports the use of an operating system (Wilensky et al., 1984), asking an inexperienced user to `Remove the folder with the system tools' could have tremendous effects on the system itself.

If one assumes a modular division of the natural language generation task between two stages

of the language production process | deciding what to say (conceptual level) and deciding how to say it (grammatical level) | it is not realistic to expect that the conceptual component will be able to specify the input for the grammatical component such that ambiguous utterances can be avoided.

If it were possible to specify the input in such a way, then this would mean that the conceptual component has to provide all information needed by the grammatical component to make decisions about lexical and syntactic choices. Hence, the conceptual component would need detailed information about the language to use. But this would blur the distinction between the grammatical and the conceptual level, because this would imply that both components share the grammar (see also Appelt (1989), Meteer (1990), Neumann (1991)).1

In order to maintain a modular design additional mechanisms are necessary to perform some monitoring of the generator's output. Several authors argue for such additional mechanisms (Jameson and Wahlster, 1982; De Smedt and Kempen, 1987; Joshi, 1987; Levelt, 1989). For example, Levelt (1989) pointed out that speakers monitor what they are saying and how they are saying it". In particular he shows that a speaker is also able to note that what she is saying involves a potential ambiguity for the hearer and can handle this problem by means of selfmonitoring.

In this paper we describe an approach for self-monitoring which allows to generate unambiguous utterances in such situations where possible misunderstandings by the user have to be avoided. The proposed method is based on a very strict integration of parsing and generation. During self-monitoring a generated ambigu-

1As pointed out in Fodor (1983) one of the characteristic properties of a module is that it is computationally autonomous. But a relevant consideration of computationally autonomy is that modules do not share sources (in our case the grammar).