This paper describes an efficient and robust implementation of a bidirectional, head-driven parser for constraint-based grammars. This parser is developed for the OVIS system: a Dutch spoken dialogue system in which information about public transport can be obtained by telephone. After a Review

In this paper I will discuss the properties of a tactical generation approach that has become popular recently: head-driven bottom-up generation. It is assumed that bidirectional grammars written in some unificationor logic-based formalism define relations between strings and some representation, usually called logical form. The task for a generator is to generate for a given logical form the strings that are related to this logical form by the grammar. In the paper it will be shown that the `early' approaches to this conceivement of the generation problem such as [21], [31] and [7] are not entirely satisfactory for general purposes. Furthermore I will define a simple bottom-up generator, called BUG1 for reference, as prototypical for the head-driven bottom-up approach as defended by for example [30, 5, 23, 24]. I will argue that head-driven bottom-up generation is to be preferred because the order of processing is directed by the input logical form and the information available in le...

this paper is that neither has been implemented." ([Vaughan and McDonald, 1986], page 95). Although Meteer [1990] gives a detail description of the relationship between text structure and revision it is unclear how the proposed model could contribute to the choice problem of paraphrases (see section 5.2). How- ever, from the approach described above and from the system described in [Meteer, 1990] we can draw the following conclusions. Only the generatoFs input is marked. If the generator encounters alternative realizations the revision component is asked to make the decision. However, to be able to do this it needs detailed knowledge about the grammar. Therefore grammatical knowledge has to be duplicated. The linguistic realization component used in [Meteer, 1990] is MUMBLE-86 [McDonald, 1986]. The text structural representation level must completely specify the infor- mation to be expressed by the utterance. The mapping has to ensure that all the necessary linguistic information is present. Mumblers procedural grammar is used only for generation purposes. Therefore it is without reach for the revision model to take into account relevant sources of ambiguities

It is well known that even extremely linited centerembedding causes people to have difficulty in comprehension, but tits[ left- and right-branching cons[factions produce no such effect. If the difficulty in comprehension is taken to be a result of processing load, as is widely assumed, then measuring the processing load induced by a parsing strategy on these constructions may help determine its plausibility as a psychological model. On this basis, it has been argued [AJ91, JL83] that by identifying processing load with space utilization, we can rule out both top-down and bottom-up parsing as viable candidates for the human sentence processing mechanism, and that left-corner parsing represents a plausible Mternative.

We propose a generalization of Categorial Grammar in which lexical categories are defined by means of recursive constraints. In particular, the introduction of relational constraints allows one to capture the effects of (recursive) lexical rules in a computationally attractive manner. We illustrate the linguistic merits of the new approach by showing how it accounts for the syntax of Dutch cross-serial dependencies and the position and scope of adjuncts in such constructions. Delayed evaluation is used to process grammars containing recursive constraints.

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I describe a head-driven parser for a class of grammars that handle discontinuous constituency by a richer notion of string combination than ordinary concatenation. The parser is a generalization of the left-corner parser and can be used for grammars written in powerful formalisms such as non-concatenative versions of UCG and HPSG.

A corpus-based technique is described to improve the efficiency of wide-coverage high-accuracy parsers. By keeping track of the derivation steps which lead to the best parse for a very large collection of sentences, the parser learns which parse steps can be filtered without significant loss in parsing accuracy, but with an important increase in parsing efficiency. An interesting characteristic of our approach is that it is self-learning, in the sense that it uses unannotated corpora. 1

The MiMo2 translation system combines several leading ideas in the areas of linguistics, computation and translation. In the area of translation we follow the ideas of Landsbergen [22] by assuming that translation is symmetric; and combine these ideas with the advantages of a transfer approach. Computationally the system focuses on computability and declarativity. The linguistics of the system is based on a lexicalistic and sign-based approach to grammar. 1 Introduction The MiMo2 system is based on a fundamental distinction between `possible' and `best' translation (Landsbergen [22]). Linguistically, a source text can have many possible translations, i.e. many target language texts that are equivalent in meaning. In practice, some interpretations will be more plausible than others, and some translations of some interpretations will often be preferred over others on various grounds, e.g. style. The MiMo2 system attempts to capture the notion `linguistically possible translation', as ...

Although unification can be used to implement a weak form of beta-reduction, several linguistic phenomena are better handled by using some form of lambda-calculus. In this paper we present a higher orderfeature description calculus based on a typed lambda-calculus. We show how the techniques used in CLG for resolving complex feature constraints can be eciently extended. CCLG is a simple formalism, based on categorial grammars, designed to test the practical feasibility of such a calculus.