SEMHE: A Generalised Two-level System

S. EMH. E: A Generalised Two-Level System George Anton Kiraz* Computer Laboratory University of Cambridge (St John's College) Email: George. KirazOcl. cam. ac. uk URL: http ://www. c1. cam. ac. uk/users/gkl05 Abstract This paper presents a generalised two- level implementation which can handle lin- ear and non-linear morphological opera- tions. An algorithm for the interpretation of multi-tape two-level rules is described. In addition, a number of issues which arise when developing non-linear grammars are discussed with examples from Syriac. 1 Introduction The introduction of two-level morphology (Kosken- niemi, 1983) and subsequent developments has made implementing computational-morphology models a feasible task. Yet, two-level formalisms fell short from providing elegant means for the description of non-linear operations such as infixation, circumfix- ation and root-and-pattern morphology} As a re- sult, two-level implementations - e.g. (Antworth, 1990; Karttunen, 1983; Karttunen and Beesley, 1992; Ritchie et al., 1992) - have always been bi- ased towards linear morphology. The past decade has seen a number of proposals for handling non-linear morphology; 2 however, none * Supported by a Benefactor Studentship from St John's College• This research was done under the super- vision of Dr Stephen G. Pulman. Thanks to the anony- mous reviewers for their comments. All mistakes remain mine. 1Although it is possible to express some classes of non-linear rules using standard two-level formalisms by means of ad hoc diacritics, e.g., infixation in (Antworth, 1990, p. 156), there are no means for expressing other classes as root-and-pattern phenomena. 2(Kay, 1987), (Kataja and Koskenniemi, 1988), (Beesley et al., 1989), (Lavie et al., 1990), (Beesley, 1990), (Beesley, 1991), (Kornai, 1991), (Wiebe, 1992), (Pulman and Hepple, 1993), (Narayanan and Hashem, 1993), and (Bird and Ellison, 1994). See (Kiraz, 1996) for a review. (apart from Beesley's work) seem to have been im- plemented over large descriptions, nor have they pro- vided means by which the grammarian can develop non-linear descriptions using higher level notation• To test the validity of one's proposal or formalism, minimally a medium-scale description is a desider- atum. SemHe 3 fulfils this requirement• It is a gen- eralised multi-tape two-level system which is being used in developing non-linear grammars. This paper (1) presents the algorithms behind SemHe; (2) discusses the issues involved in compil- ing non-linear descriptions; and (3) proposes exten- sion/solutions to make writing non-linear rules eas- ier and more elegant. The paper assumes knowledge of multi-tape two-level morphology (Kay, 1987; Ki- raz, 1994c). 2 Linguistic Descriptions The linguist provides SemHe with three pieces of data: a lexicon, two-level rules and word formation grammar• All entries take the form of Prolog terms. 4 (Identifiers starting with an uppercase letter denote variables, otherwise they are instantiated symbols•) A lexical entry is described by the term synword( <morpheme>, (category)). Categories are of the form (category_symbol) : [(f eature_attrl = value1>, <]eature_attrn = wlu n) ] a notational variant of the PATR-II category formal- ism (Shieber, 1986). 3The name SemHe (Syriac .semh~ 'rays') is not an acronym, but the title of a grammatical treatise writ- ten by the …

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