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<Paper uid="P84-1089">
  <Title>Coping with Extragrarnmaticality</Title>
  <Section position="2" start_page="0" end_page="0" type="intro">
    <SectionTitle>
1. Introduction
</SectionTitle>
    <Paragraph position="0"> Any robust natural language interface must be capable of processing input utterances that deviate from its grammatical and semantic expectations. Many researchers have made this observation and have taken initial steps towards coverage of certain classes of extragrammatical constructions. Since robust parsers must deal primarily with input that does meet their expectations, the various efforts at coping with extragrammaticality have generally been structured as extensions to existing parsing methods. Probably the most popular approach has been to extend syntactically.oriented parsing techniques employing Augmented Transition Networks (ATNs) \[21, 24, 25, 29\]. Other researchers have attempted to deal with ungrammatical input through network-based semantic grammar techniques \[19. 20j. through extensions to pattern matching parsing in which partial pattern matching is allowed \[16\], through conceptual case frame instantiafion \[12, 22\], and through approaches involving multiple cooperating parsing strategies \[7, 9, 18\].</Paragraph>
    <Paragraph position="1"> Given the background of existing work, this paper focuses on two major objectives:  1. to create a taxonomy of grammatical deviations covering a broad range of extragrammaticalities, 2. to outline strategies for processing many of these deviations, 3. to assess how easily these strategies can be employed in  conjunction with existing parsing methods.</Paragraph>
    <Paragraph position="2"> The overall result should be a synthesis of different parse. recovery strategies organized by the grammatical phenomena they address (or violate), an evaluation of how well the strategies  input, and a set of characteristics desirable in any parsing process dealing with extragrammatical input. We hope this will aid researchers designing robust natural language interfaces in two ways: t.by providing a tool chest of computationally effective approaches to cope with extragrammaticality; 2. by assisting in the selection of a basic parsing methodology in which to embed these recovery techniques.</Paragraph>
    <Paragraph position="3"> In assessing the degree of compatibility between recovery techniques and various approaches to parsing, we will avoid the issue of whether a given recovery technique can be used with a specific approach to parsing. The answer to such a question is almost always affirmative. Instead, we will be concerned with how naturally the recovery strategies fit with the various parsing approaches. In particular, we will consider the computational tractability of the recovery strategies and how easily they can obtain the information they need to operate in the context of different parsing approaches.</Paragraph>
    <Paragraph position="4"> Extragrammaticalities include patently ungrammatical constructions, which may nevertheless be semantically comprehensible, as well as lexical difficulties (e.g. misspellings), violations of semantic constraints, utterances that may be grammatically acceptable but are beyond the syntactic coverage of the system, ellipsed fragments and other dialogue phenomena, and any other difficulties that may arise in parsing individual utterances* An extragrammaticality is thus defined with respect to the capabilities of a particular system, rather than with respect to an absolute external competence model of the ideal speaker. Extragrammaticality may arise at various levels: lexical, sentential, and dialogue. This paper addresses the first two categories; the third is discussed in \[8, 11\]. Our discussions are based on direct experience with various working parsers: FLEXP, CASPAR and DYPAR \[7, 8, 16\].</Paragraph>
  </Section>
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