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<Paper uid="C88-1063">
  <Title>An Experimental Parser for Systemic Grammars</Title>
  <Section position="3" start_page="0" end_page="309" type="intro">
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    <Paragraph position="0"> -Clause ~-... MOOD TYPE a language, and may be regarded as &amp;quot;hooks&amp;quot; into a semantic component. The realization statements determine the constituent structure. There are realization statements to declare the presence of constituents, conflate constituents, specify feature constraints on constituents, and specify ordering constraints among constituents. Consider, for example, the fragment of a grammar of English clauses shown in Figure 1. There are two systems, labeled by Mood-type and Indicative-type. Each system has an input condition to its left, specifying when its options are applicable. The input condition for Indicative-type is the single feature, Indicative, but input conditions may also be expressed by boolean combinations of features. In each system, exactly one of the features to the right of the vertical bar must be chosen. For example, in the Indicative-type system, either Declarative or Interrogative must be chosen. Under each feature are realization statements, such as SUBJECT A FINITE under the Declarative feature. This statement specifies that the SUB-JECT constituent must precede the FINITE constituent in declarative clauses. Each realization statement is associated with a particular feature, so that structural constraints are distributed throughout the system network. The distributed nature of structural information in SG presents a challenge to the design of a parser, which we will address in Section 3.</Paragraph>
    <Paragraph position="1"> In addition to building a constituent structure for a sentence, as do most syntactic approaches to natural language parsing, a parser for SO must also perform the following tasks: 1. determine the set of systemic features for each constituent, 2. assign grammatical functions to each constituent.</Paragraph>
    <Paragraph position="2"> Other theories of grammar also make use of features and grammatical functions, however they have a distinct significance in systemic theory. The feature set associated with a constituent plays an important role in specifying its meaning (i.e., the features are not simply discarded after syntactic analysis), so a relatively I~,rge number (e.g., over 50) of features may need to be assigned to eac!, constituent.</Paragraph>
    <Paragraph position="3"> Each constituent may also be assigned to several grammatical functions, because of the multifunctional nature of systemic analysis. For example, it is common to describe a single constituent simultaneously as SUBJECT, ACTOR and TOPIC. Therefore, in order to determine that a clause has an ACTOR, it may be necessary to check whether the clause has a SUBJECT and whether the SUBJECT of the clause is conflatable with the. ACTOR function.</Paragraph>
    <Paragraph position="4"> An example of the type of output produced by the parser is shown in Figure 2. This example shows only the functional structure that the parser assigns to the sentence. In addition, each constituent is assigned a set of grammatical features, such as Indicative and Declarative. These features are also accessible in the data structures produced by the parser, but they are too numerous to display in this short paper.</Paragraph>
    <Paragraph position="5">  The basic method used to construct the parser has been to develop a compiled representation of systemic grammars in the notation of Functional Unification Grammar (FUG). The parsing process itself is then derived by extending methods already developed for parsing With FUG \[Kay 85\]. In FUG, a grammar can be regarded as a special kind of logical formula \[Rounds 87\], and the parsing problem is reduced to finding the set of feature structures that satisfy the formula subject to the constraints of the words in a particular sentence. Using the feature description logic (FDL) of Kasper and Rounds \[Kasper 86\], the types of formula used to define a grammar include: 1 NIL denoting no i~formatloa; a where a E A, to describe atomic values; l : ~b where I E L and ~ E FDL~ to describe structures in which the feature labeled by l has a value described by ~; ql or l : ANY where l E L, to describe a sliructure in which I has a substantive (non-NIL I value; &lt; p &gt; where p E L*, to describe a structure that shares a common value with the path p; \[~bl ... ~\] where ~b~ E FDL, denoting conjunction; {~bl ... ~b,~} where ~b~ E FDL, denoting disjunction; ~1 --* ~ where ~b~ E FDL, denoting classical implication.</Paragraph>
    <Paragraph position="6"> The last type of formula, denoting implication, is an extension to FUG that enables a more efficient modeling of systemic descriptions than is possible in Kay's version of FUG IKasper 87d\].</Paragraph>
    <Paragraph position="7"> The compilation of systems into FUG is relatively straightforward. Each system is represented by a disjunction containing alter. natives for each feature that can be chosen in the system. These alternatives also contain attributes that represent constraints on grammatical functions imposed by realization statements. For example, the Mood-type and Indicative-type systems can be represented by the description shown in Figure 3. System input conditions are bidirectional: they are represented by the embedding of descriptions, and also by feature existence conditions.</Paragraph>
    <Paragraph position="8"> In the FUG representation there is one functional description (FD) corresponding to each m~ior constituent category of the systemic grammar. Major constituent categories for English include clause, nominal-group, and prepositional-phrase. The method of representing a systemic grammar as a set of FDs in FUG is described in greater detail in \[Kasper 87b,Kasper 87d\]. A program has been implemented to automatically translate any system network into FDs, verifying the effectiveness and generality of this compilation procedure. This program has been used to compile the entire Nigel grammar, which contains over 500 systems, into FUG many different times as changes to the grammar have been made.</Paragraph>
    <Paragraph position="9"> :Let A and L be sets of symbols used to denote atomic valueJ and feature labels, respectively.</Paragraph>
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