• Marcu has characterised an important and difficult problem in text planning: given a set of facts to convey and a set of rhetorical relations that can be used to link them together, how can one arrange this material so as to yield the best possible text? We describe experiments with a number of heuristic Search methods for this task. ' Text Planning This paper presents some initial experiments using stochastic search methods for aspects of text planning. The work was motivated by the needs of the ILEX system for generating descriptions of museum artefacts (in particular, 20th Century jewellery) [Mellish e t al 98]. We present results on examPles semi-automatically generated from datastructures that exist within ILEX. Forming a set of facts about a piece of jewellery into a structure that yields a coherent text is a non-trivial problem. Rhetorical Structure Theory[Mann and Thompson 87] claims that a text is coherent just in case it can be analysed hierarchically in terms of relations between text spans. Much work in NLG makes the assumption that constructing something like an RS tree is a necessary step in the planning of a text. This work takes as its starting point Marcu's [Marcu 97] excellent • formalisation of RST and the problem of building legal RST trees, and for the purposes of this paper the phrase "text planning" will generally denote the task characterised by him. In this task, one is given a set of facts all of which should be included in a text and a set of relations between facts, some of which canbe included in the text. The task is to produce a legal RS tree using the facts and some relations (or the "best" such tree). Following the original work on RST and assumptions that have been commonly made in subsequent work, we wil ! assume that there is a fixed set of possible relations (we include "joint" as a second-class relation which can be applied to any two facts, but whose use is not preferred). Each relation has a nucleus and a satellite (we don't consider multiple nuclei or satellites here, apart from the case of "joint", which is essentially multinuclear). Each relation may be indicated by a distinctive "cue phrase", with the nucleus and satellite being realised in some fashion around it. Each relation has applicability conditions which can be tested between two atomic facts. For two complex text spans, a relation holds exactly when that relation holds between the nuclei of those spans. Relations can thus hold between text spans Of arbitrary size. Figure 1 shows an example of the form of the input that is used for the experiments •reported here. Each primitive "fact" is represented in terms of a subject, verb and complement (as well as a unique identifier). The "subject" is assumed to be the entity that the fact is "about". The approaches reported here have not yet been linked to a realisation component, and so the entities a 80 South Bridge, Edinburgh EH1 1HN.
REF investigate the problem of determining a discourse tree for a set of elementary speech acts which are partially constrained by rhetorical relations.
Experiments Using Stochastic Search For Text Planning
Workshop On Natural Language Generation EWNLG
Computer Science