Extended Access to the Left Context in an ATN Parser

EXTENDED ACCESS TO THE LEFT CONTEXT IN AN ATN PARSER Irlna Prodanof and Giaeomo Ferrari Istltuto di Linguistica Computazionale Via della Faggiola 32 1-56100 Pisa, Italy ABSTRACT Some Italian sentences related to linguistic phenomena largely known and recently discussed by many computational linguists are discussed in the framework of AT:~. They offer certain difficulties which seem to suggest a substantial revision of the ATN formalism. The theoretical assumptions and an experimental implementation of such a revision are presented, together with examples. }:any related theoretical points such as some psyeholinguistic implications and the relationship between deterministic and non-deterministlc hypothesis are also briefly discussed. I. INTRODUCTION Certain types of sentences seem to defy the abilities of several parsers, and some of them are being now discussed by many computational linguists, mostly within the deterministic hypothesis. An exmaination of their treatment within the traditional AT[: paradigm seems to suggest that the real discussion is about how to acces the left context and what its form should be. II. ACCESS TO T}]E LEFT COfJTEXT occurs, the parsing algorith~ suspends the current computation and starts a new level of computation. Usually, each network recognizes some linguistic unit such as Noun Phrase (NP), Prepositional Phrase (PP), and Sentence (S) and any recursively embedded call to one of them corresponds to a level of computation. The parsed parts of the input string are stored (SETRed) into registers as they are recognized. At the end of the network these registers are combined (BUILDQed) into a parse node and returned (POPed) to the calling net. Appropriate functions can return the content of a register (GETR) or transfer it to another register (combination of SETR with a GETR). This last operation is equivalent to i) the renaming of a register, if the source register is successively set to a different value, ii) the initialization of a register at a lower or higher level, if SENDR or LIFTR are used. Initialization is co~aonly used to i) raise lexical features to a higher level where they are used for tests (ex.: subject-verb agreement), ii) pass possible antecedents to lower levels where a gap may be detected in an embedded clause. B. Difficult access to registers i. Filler-gap linking The antecedent passing theoretically unlimited increase By the standard procedure, the m~biguous sentence(1) may cause a in storage load. analysis of the (I) Giovanni disse che aveva mentito John said that (he) had lied A. ATN Grammars "Giovanni" is always SENDRed as possible SUBJect of a complement, as soon as "disse" is recognized An ATN grammar is a set of networks formed by as an STF~ENS verb. As no subject NP is met after labelled states and directed arcs connecting them. "che', an interpretation is yielded with The arcs can rlco~nize terminal (words) and "Giovanni" in subject position. The second non-temnlnal (lexical cateF, ories) s~anbols or interpretation is produced si,,ply by successively reeursively call for a network identified by the setting the SULqJ register to a d~;~my node, which label of an initial state. …

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