1 Introduction: an application experiment - Semantic Scholar

Using text structure and text planning to guide text summarization John A. Bateman1

GMD-IPSI, Dolivostr. 15, D-64372 Federal Republic of Germany

1 Introduction: an application experiment This paper will brie y outline an ongoing experiment being carried out at the GMD institute IPSI in Darmstadt, concentrating on the text generation aspects of the experiment and their possible relation to `summarization'. The experiment combines work from three distinct areas: the `editor's workbench' under development by the pave-group within IPSI as part of the European Community funded RACE-project europublishing, the text analysis component kontext developed by the kontext-group at IPSI, and the text generation component komet-penman(ml) (kpml) under development by the komet group at IPSI with input from University of Sydney. The experiment envisages the following scenario. An editor of a large-scale publication is gathering information from many source articles, considering how they are to be presented, what overview information can be given, etc. The editor's workbench supports this work by providing a graphically oriented object network editor, where objects can be displayed in a variety of styles. This workbench is implemented and has already been very favorably received by some potential end-users. The current application domain of the workbench is the `Dictionary of Art': a large publication being prepared by MacMillan publishers. The functionality of this workbench is now being augmented. In particular, we include:  

deep analysis of incoming articles, producing a semantic representation of the content, presentation of that semantic representation in textual form.

These functionalities give rise to `summarization' of various kinds, although summarization itself is not targetted as an independent task. This raises a number of issues concerning summarization: e.g., is summarization an independent task? Do `summaries' have particular linguistics properties that need to be captured independently of other types of texts? Is their any dierence between `summarization' and, e.g., text generation as a whole?|since text generation is always of necessity selection of information to be expressed and cannot assume that that information is anything but a `summary' of the total information that could be expressed. 1

Also on extended leave from USC/ISI, Los Angeles.

2 Some dierent kinds of summarization in the scenario The places in our experiment where something similar to `summarization' functionalities can be found are as follows. The approach of the text analysis system is to provide deep semantic modelling of selected linguistic elds: e.g., the eld of change-of-possession, motion, creation, etc. This deep semantic modelling is carried out on the basis of work such as [Kunze, 1991, Kunze, 1993], which now has received computational implementation [Firzla and Haenelt, 1992]. The construction of objects in the editor's workbench object network therefore proceeds by picking out of texts analyzeable sentences concerning one or more of the already handled semantic elds. In the present experiment, sentences concerning `creation' have been targetted. During the inputs of source article texts, The constructed semantic network grows by addition of all facts concerned with creations of art objects, buildings, etc. We thus have a ` ltering' eect on the input, which can also be interpreted as internal summarization with respect to a selected-topic. The editor's workbench itself supports graphical navigation according to the user's interaction with the object network. At any time, not all of the network is in view and certain types of relations (speci ed by or for the end-user) may be visible (view `styles'). This oers `summarization' of the local contents of the object network according to the user's immediate interest. Finally, the editor's workbench may pass a request to the text generation component for the generation of a natural language textual expression of information in the object network. In addition to the constraints of what information is to be found in the object network, and the starting point of local interest xed by the user during graphical navigation, the architecture of the text generation component also enforces a kind of summarization behavior since it seeks to strongly constrain the information that will be utilized in any text. This is to ensure that the text generation process is not overrun by the information to be expressed. The size of the object network containing information can be expected to grow explosively over the next year: the text generation process has, therefore, to bring to bear powerful constraints for restricting the information that it needs to access. This process probably comes closest to what is usually meant by summarization.

3 The text generation architecture The current architecture of kpml attempts a full implementation of a systemically organized natural language architecture. In the spirit of, for example, [Cross, 1992], the systemic organization of [Halliday, 1978, Matthiessen, 1992, Martin, 1992] is used as the basis for all levels of linguistic information in the system|including morphology, grammar, discourse semantics, register and genre. This computational architecture is under development with input from a number of cooperative research projects; these include on-

Text Type / Genre ‘goals’ ‘generic structure (stages)’

‘contextual domain knowledge’

Discourse Semantics motivation

T

belief spaces

R

attitudes T T

R R

‘thematic progression’

elaboration

background

information status

elaboration

‘rhetorical structure’

‘semantic ontology’

‘audience model’

‘lexico–grammar’

Figure 1: KOMET-PENMAN(ML) architecture going work on `register' (e.g., [Bateman and Paris, 1990]), work on multilinguality in text generation [Bateman et al., 1991, Bateman et al., 1993], and work in the EC-funded basic research project dandelion (EP6665: Commission of European Communities). The architecture represents a natural extension of work such as that of [Hovy et al., 1992], moving in the same spirit towards a theoretically more homogenous treatment of textual phenomena. In the present context, what is central concerning the architecture is its reliance on `text type', or genre. Text generation only begins once a text type has been selected. This selection brings with it constraints both on the type of information to be selected from the knowledge base and its overall ordering into `generic stages' to be presented over the text. This level of description corresponds exactly to the schemata originally used in [McKeown, 1985]. However, as suggested by the systemic orientation, schema are not unanalyzed wholes but are themselves the consequences of a classi cation hierarchy of a similar type to that deployed in the grammar. Classifying the genre gives rise to constraints on the generic structure that appears, just as classify within the grammar gives rise to constraints on the syntactic structure that appears. Each generic stage is then realized further by classi cation by the discourse semantics: a level of organization treated in depth in [Martin, 1993]. The result of this is a sequence of `micro-semantic' speci cations that can be passed to a systemic grammar: in our case, systemic grammars of German, English (the Nigel) grammar, and Dutch. The architecture is shown in Figure 1. In our present work, we have been focusing on biographies of various kinds. Such texts have a relatively stable structure at the level of generic organization. Examples of texts recently generated are:

-- Behrens began his professional career as a painter. -- He attended the art schools in Duesseldorf and Karlsruhe in 1886 - 1889. -- He studied in Munich in 1890 with Kotschenreiter. -- He gave up painting after 1900. -- He took up architecture in Darmstadt.

or -- Behrens's principal activities were architecture and industrial design. -- He made electrical appliances and flasks for mass production in a glass works. -- Behrens started his career in Darmstadt in 1899 as an architect. -- He built the high tension plant and the turbine factory for AEG in 1908 - 1910. -- He built a housing area for the workers of AEG in Henningsdorf. -- He created a number of monumental buildings and the German embassy in St. Petersburg.

with equivalents for German and Dutch. Importantly, these texts were generated entirely on the basis of the object network in the editor's workbench knowledge base. The text planning is still very simple: for example, the granularity of facts in the knowledge base is largely taken over in the granularity of the events presented in the text. But this is in no way hardwired in the architecture; all such `decisions' are consequences of the classi cation variations represented in systemic networks at the various levels of abstraction supported.

4 Summarization? The question can then be raised, are the above texts and those like it `summaries'? In a certain sense they clearly are: although they are not summaries of particular texts. They gather information from the knowledge base in order to ll out a text that has the structure and content of a subtype of biographies. To the extent that a biography can be said to be a summary, then these are summaries. But the same can probably be said regardless of the kind of text we generate. The same processes will be followed. Is there then a separate kind of text type that we can label `summary'? Our work on biographies proceeds as follows. First, analyses of biography texts are undertaken to yield a `register pro le' of the text type. The function of biographies is also classi ed against a growing network of genre-level options. This needs also to be done for texts that would be described as summaries. However, whether there is additional information arising out of the fact of being a summaries that would not already be subsumed elsewhere is unclear. This can only be answered by empirical studies. If there is such an identi able text type, then we can enter it into our genre level resources just as with any other text type. What is important is that text level constraints on the content and form of texts are an integral part of the architecture. As long as that is acheived, then text generation mechanisms would appear a natural candidate for constructing summaries.

References [Bateman and Paris, 1990] John A. Bateman and Cecile L. Paris. Constraining the development of lexicogrammatical resources during text generation: towards a computational instantiation of register theory. In Eija Ventola, editor, Recent Systemic and Other Views on Language. Benjamins, Amsterdam, 1990. [Bateman et al., 1991] John A. Bateman, Christian M.I.M. Matthiessen, Keizo Nanri, and Licheng Zeng. The re-use of linguistic resources across languages in multilingual generation components. In Proceedings of the 1991 International Joint Conference on Arti cial Intelligence, Sydney, Australia, volume 2, pages 966 { 971. Morgan Kaufmann Publishers, 1991. [Bateman et al., 1993] John A. Bateman, Liesbeth Degand, and Elke Teich. Multilingual textuality: Some experiences from multilingual text generation. In Proceedings of the Fourth European Workshop on Natural Language Generation, Pisa, Italy, 28-30 April 1993, pages 5 { 17, 1993. Also available as technical report from GMD/Institut fur Integrierte Publikationsund Informationssysteme, Darmstadt, Germany. [Cross, 1992] Marilyn Cross. Choice in text: a systemic approach to computer modelling of variant text production. PhD thesis, School of English and Linguistics, Macquarie University, Sydney, Australia, 1992. [Firzla and Haenelt, 1992] Beate Firzla and Karin Haenelt. On the acquisition of conceptual de nitions via textual modelling of meaning paraphrases. In Proceedings of the fteenth International Conference on Computational Linguistics (COLING-92), volume IV, pages 1209 { 1213. International Committe on Computational Linguistics, 1992. [Halliday, 1978] Michael A.K. Halliday. Language as social semiotic. Edward Arnold, London, 1978. [Hovy et al., 1992] Eduard Hovy, Julia Lavid, Elisabeth Maier, Vibhu Mittal, and Cecile Paris. Employing knowledge resources in a new text planner architecture. In Proceedings of the 6th International Workshop on Natural Language Generation, Trento, Italy, 1992. Springer-Verlag. [Kunze, 1991] Jurgen Kunze. Kasusrelationen und semantische Emphase, volume XXXII of Studia Grammatica. Akademie Verlag, Berlin, 1991. [Kunze, 1993] Jurgen Kunze. Sememstrukturen und Feldstrukturen, volume XXXVI of Studia Grammatica. Akademie Verlag, Berlin, 1993. [Martin, 1992] James R. Martin. English text: systems and structure. Benjamins, Amsterdam, 1992. [Martin, 1993] James R. Martin. English Text: system and structure. John Benjamins, Amsterdam, 1993. [Matthiessen, 1992] Christian M.I.M. Matthiessen. Lexicogrammatical cartography: English systems. Technical report, University of Sydney, Linguistics Department, 1992. Ongoing expanding draft. [McKeown, 1985] Kathleen R McKeown. Text Generation: Using Discourse Strategies and Focus Constraints to Generate Natural Language Text. Cambridge University Press, Cambridge, England, 1985.