Tree adjoining grammar and the reluctant paraphrasing of text

Paraphrase, as a general concept, is a very mutable thing; at one extreme, paraphrase is the relation between units of text with identical meanings, as under the transformationalgenerative view, and at the other extreme there is the view that no text is interchangeable with any other. Each point in the spectrum ranging between these two views is a valid one, and in order to focus on a speci c point it is necessary to examine paraphrase under a particular context and application. In this thesis, the context is the task of Reluctant Paraphrase (RP). Given an ideal starting text, which expresses exactly what its author intends, it is often necessary to alter the text in some way in order to t it to some externally imposed constraints|length, readability as speci ed by a house style guide, and so on. In this context, paraphrase is a relation between interchangeable units of text; the task is then to choose the best set of such paraphrases that will t the whole text to the constraints, with `best' in this case being the set that changes the text the least. This di ers from other tasks where paraphrase is central: in natural language generation, particularly revision-based generation, where positively correlated constraints have meant there is no need for advanced search strategies to nd the best solution, and where the starting point is a knowledge representation rather than text, with the system consequently being omniscient regarding equivalence of concept realisations into text; and in style checkers and controlled language checkers, where the goal is to correct text, and consequently where the locality of alterations again means that advanced strategies for nding the best solution are unnecessary. The thesis uses the application of paraphrase as a means of exploring properties of the Tree Adjoining Grammar (TAG) formalism. TAG is a mathematical formalism, within which linguistic theories can be expressed, that is constrained in expressive power, which means that formal results about the language can be established, and that a linguistic theory has less work to do in circumscribing the class of acceptable constructions, since the formalism imposes universal constraints. And, as one of a class of mildly context sensitive grammars, it is well suited to describing natural language. Moreover, it is possible to map between TAG grammars using the Synchronous TAG (S-TAG) formalism, which has been used in applications such as machine translation, syntax{semantics mapping and modelling of coordination. However, in these applications the S-TAG formalism is not able to model all the phenomena that occur. Paraphrase is another, di erent application that allows exploration of the formalism from a di erent perspective, one which is particularly structurally complex. This application and the context of RP are closely interrelated. In performing the RP task, a representation of paraphrase is necessary, for which S-TAG provides a formalism. And, in using paraphrase to explore TAG, the RP context provides a focus narrowing down the paraphrases and consequently the requirements of the representation formalism. The thesis then looks at the two related modelling tasks. Firstly, the modelling of the RP task. Integer Programming is shown to be an appropriate framework, allowing a text tting the constraints to be determined, one minimally altered by paraphrasing. A range of model variants is possible, and the thesis investigates which is the best model in terms of search criteria. Secondly, the modelling of paraphrase in S-TAG. The relative structural complexity gives insights into the cause of representational di culties with S-TAG, and leads to generalisations of S-TAG that resolve existing problems in the formalism.