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<Paper uid="P95-1056">
  <Title>A Minimalist Head-Corner Parser</Title>
  <Section position="4" start_page="338" end_page="339" type="metho">
    <SectionTitle>
3 Head-corner parsing
</SectionTitle>
    <Paragraph position="0"> The main idea behind head-driven parsing (Kay, 1989) is that the lexical entries functioning as heads contain valuable information for the parsing process.</Paragraph>
    <Paragraph position="1"> For example, if a verb is intransitive it will not require a complement, if it is transitive it will require a complement. Therefore the head is parsed before its sisters in a head-driven parser. A head-corner parser (Kay, 1989; Bouma and van Noord, 1993) is a special type of head-driven parser. Its main characteristic is that it does not work from left to right but instead works bidirectionally. That is, first a potential head of a phrase is located and next the sisters of the head are parsed. The head can be in any position in the string and its sisters can either be to the right or to the left.</Paragraph>
    <Paragraph position="2"> A head-corner parser starts the parsing process with a prediction step. This step is completed when iSee (Veenstra, 1994) for further details.</Paragraph>
    <Paragraph position="3"> a lexical head is found that is the head-corner of the goal (i.e. the type of constituent that is parsed). The head-corner relation is the reflexive and transitive closure of the head relation. A is the head of B if there is a rule with B as left hand side (LHS) and A as the head daughter on the right hand side (RHS).</Paragraph>
    <Paragraph position="4"> When a (lexical) head-corner is found an X rule is selected in which the (lexical) head is on the RHS.</Paragraph>
    <Paragraph position="5"> The sisters of the head are parsed recursively. The LHS of the rule contains the mother of the head.</Paragraph>
    <Paragraph position="6"> If this mother is a head-corner of the goal, and the mother and the goal are not equal the whole process is repeated by selecting a rule with the new head-corner (i.e. the mother of the first head-corner) on its RHS.</Paragraph>
    <Paragraph position="7"> In section 2 it is assumed that movement is invariably leftward and that GT and Move-a are bottom-up mechanisms. GT builds the VP before other projections. Constituents of VP are moved to higher projections by Move-a, which is a special kind of GT. Suppose that the parser should consider AgrS as the head-corner of AgrSP, which accords with X-Theory. Then the head (AgrS) that should be filled with an adjoined verb by movement from AgrO (in a transitive sentence) or V (in an intransitive sentence) is created before AgrO and V. To avoid moving constituents from a part of the tree that has not been built yet, the head-corner table for the minimalist head-corner parser is not constructed completely according to X-Theory (see (1)).</Paragraph>
    <Paragraph position="8">  (1) hc(AgrS,AgrSP), hc(V,VP).</Paragraph>
    <Paragraph position="9"> hc(AgrOP, AgrS). hc(V,V).</Paragraph>
    <Paragraph position="10"> hc(AgrO,AgrOP), hc(N,NP).</Paragraph>
    <Paragraph position="11"> hc(VP, AgrO). hc(N,~).</Paragraph>
    <Paragraph position="12">  For example, instead of AgrO, VP is the head-corner of AgrO. This solution is compatible with the Minimalist Program in the sense that in this way the tree is built up in an absolute bottom-up way (i.e. starting from V) so that a position that should be filled by movement is always created after the position from which the moved element comes.</Paragraph>
    <Paragraph position="13"> The head-corner table in (1) illustrates that functional heads like AgrO and AgrS are not processed as heads. Lexical proj_.ections like VP and NP are treated according to X-Theory. If we follow (1) in combination with the tree in figure 1 we establish the fact that the parser searches its way down to the verb as soon as possible. The top-down prediction step moves from thegoal AgrSP to AgrS to AgrOP to AgrO to VP to V and finally to the lexical head-corner V where the bottom-up process starts as the Minimalist Program requires.</Paragraph>
    <Paragraph position="14"> The head-corner parsing algorithm and the  structure-building operations of the Minimalist Program (GT and Move-a) have much in common. In both cases a tree is built up in a bottom-up way by starting with a head (lexical head-corner in the parsing algorithm, target in the structure building operations) and creating the sister of the head recursively, etc. 2 By treating only lexical heads as head-corners we achieved that our parsing algorithm completely represents GT. Only for Move-a we need an extra predicate to accomplish a movement if there is a possible movement to the node that has just been created.</Paragraph>
  </Section>
  <Section position="5" start_page="339" end_page="339" type="metho">
    <SectionTitle>
4 Parsing vs. Generation
</SectionTitle>
    <Paragraph position="0"> In section 3 we chose not to consider functional heads as head-corners. This choice was made because it allows GT and Move-a to start constructing a VP before the projections to which constituents from VP are moved are constructed. Another motivation to start with VP is that V contains information that is useful for the remainder of the structure building process. For example, if the verb is intransitive we know that V does not require a complement sister, and we know that we do not need an AgrOP on top of VP. The fact that V contains lexical information and functional heads like AgrO and AgrS do not, could be used as a justification for the fact that the latter are not head-corners. The main idea of head-driven parsing is, as was stated before, that heads contain relevant information for the parsing process, and that they therefore should be parsed before their sisters. Functional heads obtain their contents via movement of elements from positions lower in the tree. This special status makes them less useful for the parsing process.</Paragraph>
    <Paragraph position="1"> The Minimalist Program is a generation-oriented framework. Because we are dealing with parsing (as opposed to generation) in this paper there are certain discrepancies between the parser and the framework it is based on. In the minimalist framework, lexical information belonging to a chain is available from the moment that the first position of the chain is created, because that is the moment when the lexicon is consulted. When parsing a sentence the lexicon is not by definition consulted at the beginning of the chain. Figure 1 shows a tree that contains traces and visible constituents. The position containing a visible constituent is the SO position of that chain.</Paragraph>
    <Paragraph position="2"> The parser consults the lexicon at the moment in which the SO position of a chain is reached. Conse-Sin the minimalist head-corner parser that is described here a head always has only one sister because minimalist trees are at most binary branching.</Paragraph>
    <Paragraph position="3"> quently, when a trace is created before SO, the features belonging to that trace are unknown. The features of the traces of a certain chain are known as soon as the SO position is reached, because all positions in a chain are linked.</Paragraph>
    <Paragraph position="4"> It can be concluded that the absolute bottom-up approach for the building of trees is more useful for generation than for parsing. In generation, lexical information can be used as soon as a position that is the beginning of a chain is created. In parsing we will have to wait until the SO position is reached.</Paragraph>
    <Paragraph position="5"> In spite of this, we chose not to consider functional heads as heads in order to accomplish an absolute bottom-up process. The reason for this is that, as was mentioned before, otherwise we would be reasoning backwards with relation to movement. This could be inefficient and it is too far removed from the ideas of the minimalist framework.</Paragraph>
  </Section>
  <Section position="6" start_page="339" end_page="339" type="metho">
    <SectionTitle>
5 Future Plans
</SectionTitle>
    <Paragraph position="0"> The parser described here can judge the grammaticality of simple declarative transitive and intransitive sentences, and of subordinate clauses. We will extend the parser in such a way that it will cover more advanced linguistic phenomena like anaphors and wh-questions. Furthermore other types of parsers will be built to determine if this 'lexical' head-corner parser is indeed more efficient.</Paragraph>
  </Section>
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