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<Paper uid="C90-2050">
  <Title>A Head-Driven Approach to Incremental and Parallel Generation of Syntactic Structures</Title>
  <Section position="3" start_page="0" end_page="0" type="metho">
    <SectionTitle>
3. Overview of POPEL-HOW
</SectionTitle>
    <Paragraph position="0"> POPEL-HOW \[Fiekler&amp;Neumann89\] is the how-to-say component of the natural language generation system POPEL \[Reithinger88\]. The main features of POPEL.-HOW are: Incremental generation: POPEL-WHAT (the what-to-say component) immediately passes segments of the conceptual structure as input to POPEL-HOW when they are considered relevant for the contents to be produced. POPE1,-HOW tries to build up the corresponding semantic and syntactic segments immediately, too, in order to utter the input segments as fast as possible, i.e. POPEL-HOW generates structures at each level in an incremental way.</Paragraph>
    <Paragraph position="1"> Feedback: It is possible that new conceptual segments cannot be uttered directly because they lack necessary linguistic information. In this case POPEL-HOW is able to interact with POPEL-WHAT to demand the missing information. The bidirectional flow of control has two main advantages: Firstly, the determination of the contents can be done on the basis of conceptual considerations only.</Paragraph>
    <Paragraph position="2"> Therefore, POPEL-HOW isflexible enough to handle underspecified input 3. Secondly, POPEL-WHAT has to regard feedback from POPEL-HOW during the computation of the further selection process. This means, an incremental system like POPEL can model the influence of linguistical restrictions on the process that determines what to say next (cf. \[Hovy87\], \[Reithinger88\]).</Paragraph>
    <Paragraph position="3"> Reformulations: The addition of new conceptual segments could result in constructing semantic or syntactic segments that stand in competition with previously computed segments. POPEL-HOW is able to handle such reformulations.</Paragraph>
    <Paragraph position="4"> Unified parallel processing: Every segment at each level is conceived as an active and independent process. This view is the foundation of the parallel and incremental generation with feedback at each level of 2In our formalism we neexl only downwm'd and upward expansion because of the nature of the syntactic structures.</Paragraph>
    <Paragraph position="5"> 3\[V/ard88\] shows that most generation systems lack this ability so that their inputs have been tailored to determine a good senmncc.</Paragraph>
    <Paragraph position="6"> 2 289 POPEL-HOW. The processing approach is uniform since every process (either conceptual, semantic or syntactic) runs the same basic program and processes its segment in the same way.</Paragraph>
  </Section>
  <Section position="4" start_page="0" end_page="290" type="metho">
    <SectionTitle>
4. The Design of the Syntactic Level
</SectionTitle>
    <Paragraph position="0"> The syntactic level in POPEL-HOW is divided into two sublevels: the dependency-based level (DBS-!evel) and the level of inflected and linearized structures (ILS-level). At the DBS-level the syntactic structure is constructed in an incremental and parallel way. At the ILS-level there exists one process which performs inflection and linearization for every incoming segment of the DBS-level.</Paragraph>
    <Section position="1" start_page="0" end_page="290" type="sub_section">
      <SectionTitle>
4.1 Representation
</SectionTitle>
      <Paragraph position="0"> The central knowledge source of both sublevels is POPELGRAM, a unification-based head driven grammar. The grammar is declaratively expressed in a modified version of the PATR formalism (cf. \[Shieber85\]). PATR is modified in the sense that the representation and derivation devices are separated. To use it for generation, the parser (i.e. the derivation device) is replaced by operations suitable for incremental and parallel generation 4.</Paragraph>
      <Paragraph position="1"> POPELGRAM is divided into immediate dominance (ID) and linear precedence (LP) structures s. In order to allow for vertical orientation, the D-structures are furthermore divided into those that describe basic syntactic segments and others that describe the relationship between complex structures. Basic segments are phrases that consist of a (nonempty) set of constituents. At least one constituent must have a lexical category. The central point of view for the description of basic segments is that the dependency relation is defined for the constituents of a phrase: One constituent that must have a lexical category is denoted as the head of a phrase. All other constituents are denoted as complements that are immediately dependent on the head. The dependency relation of the constituents of a phrase is expressed as a feature description named STRUCT which belongs to the phrasal category. E. g., fig. 1 shows the feature set of the basic segment for constructing a sentence  more detail.</Paragraph>
      <Paragraph position="2"> phrase where the head governs two  -head: Fi\]. 1 comp, Ii lf t:s&amp;quot;bJl l sub, at&amp;quot; / :lvm: N J / * t2. rfct: dirobj\] / \[ &amp;quot;\[va :N J\] syn: \[~Ilocal: \[agree: ~\]\] \] 2: \[\]cat: NP1 I 11  fset: syn: agree: FYJ-pers:\[\]\] te-J.num: \[ \]J r~cat: NP2 3: te4fset: \[syn: \[case: Ace\] \] \] Fig. 1 The head element is the central element of the phrase and determines the characteristic properties of the whole phrase that is defined as the projection of its head 7. The particular quality of ID-structures of POPELGRAM makes it possible to interpret such structures as theoretically based on dependency grammar (cf. \[Kunze75\], \[Hellwig861). Basic segments can also be defined as abstract descriptions of certain classes of lexical elements which have the same category and valence (or subcategorization) restrictions. The abstract description of lexical information (in the sense of dependency grammar) is the foundation of our lexically based grammar. We assurne that this view supports incremental and parallel generation.</Paragraph>
      <Paragraph position="3"> Obviously, basic segments build up syntactic constraints for their complements* Although this seems to emphasize horizontal orientation, these constraints are not redundant and therefore do not violate the view of vertical orientation. A basic segment must access the information that is necessary to build up a  elements in the corresponding ID-rule S ~ V, NP1, NP2. Note: The order of constituents is not assumexl to be the order of the corresponding surface string.</Paragraph>
      <Paragraph position="4"> 7If complements of basic segments are defined as phrases, then the head elements of the complements are immediately dependent on the head element of the basic segment, because of the projection principle. Hence, complex syntactic structures are compositionally viewed as hierarchical head and modifier su~actures.  minimal syntactically well-formed partial utterance. If the dependencies between a head element and its modifiers are strong as in the case of complements, then this information has to be formulated in an incremental grammar, \[oo 8.</Paragraph>
      <Paragraph position="5"> Adjuncts are not necessary for building minimal well-formed structures. Therefore they are not part of basic segments. The combination of basic segments and adjuncts is expressed by means of complex segments. Those ID-structures are applied only when the corresponding basic segment already exists.</Paragraph>
    </Section>
    <Section position="2" start_page="290" end_page="290" type="sub_section">
      <SectionTitle>
4.2 Incremental and Parallel Processing
</SectionTitle>
      <Paragraph position="0"> l~D-structures are processed at the DBS-level.</Paragraph>
      <Paragraph position="1"> At this level basic segments are considered as independent and active units. In order to realize this every basic segment is associated with its own active process (or object) whereby the ,;tructure of an object reflects the underlying dependency relation of the segment. For the feature set of fig. 1 this can graphically be represented as follows: \[f,q s~mtcr: Fig. 2 (v~ is denoted as the body and the directed NP1 NP2 labeled slots 0 &amp;quot;9~ and @ as the context of the process. The names of the body and the slot correspond to the feature sets of the associated basic segment. Every labeled slot serves as a communication link to a process which represents the basic segment of the corresponding slot's complement. The topology of the network of active processes represents the corresponding dependency-based ~ree structure.</Paragraph>
      <Paragraph position="2"> Objects at the DBS-level are created during the evaluation of local transition rules. These  branch only by one element at a time.</Paragraph>
      <Paragraph position="3"> rules describe the relation between semantic and syntactic knowledge in a distributed way \[Finkler&amp;Neumann89\]. They are local because every rule describes tile mapping of semantic segments (represented as concepts of a network-like formalism) to syntactic segments of the ID-part of POPELGRAM. In principle this local mapping is case-frame based: A semantic head (e.g., a predicate) and its deep cases (e.g., agent or benefactive) am related to a corresponding syntactic head (e.g. a verb) and its syntactic frame (e.g., subject or direct object constituents). During the mapping lexical material which is deteIxnined through the choice of words 9 directs and restricts the choice of possible syntactic structures.</Paragraph>
      <Paragraph position="4"> In order to construct syntactic structures in an incremental and parallel way, every DBS-object has to solve the following two central subtasks: a. building up connections to other objects b. mapping itself as fast as possible into the next level In order to solve task a., every new created DBS-object tries to integrate itself into the topology of the already existing objects.</Paragraph>
      <Paragraph position="5"> Thereby the following cases have to be distinguished: I. The new object is taken up into the context of an object. Syntactically, this means that tile new object represents an immediately dependent constituent. The existing structure is expanded downward (see fig. 3).</Paragraph>
      <Paragraph position="6">  The new active object is the left one.</Paragraph>
      <Paragraph position="7">  into semantic segments, the content words - e.g. verbs and nouns - ,are selected. Tile selection of function words - e.g. prepositions which depend on the meaning of tile verb - is performed during the mapping between the semantic level and the DBS-level.</Paragraph>
      <Paragraph position="8">  In this example the new verbal object binds two existing nouns.</Paragraph>
      <Paragraph position="9"> 3. The new object describes a reformulation of the basic segment of an already existing object. This object must be replaced with the new one (see fig. 5).</Paragraph>
      <Paragraph position="10">  This is actually uttered as: &amp;quot;Peter loves Susan ... uh ... Mary.&amp;quot; While new communication links are built up, participating objects exchange their syntactic features 'deg. An adjunct is added to an object by augmenting the object's context with a new labeled slot. When the slot is being filled with an object, its ass~iated feature set and the basic segment are combined to yield a complex segment that represents the new segment of the augment~ object.</Paragraph>
      <Paragraph position="11"> Every DBS-object tries to map itself into the next level as fast as possible (subtask b.) in 10 This is performed with the help of a search algorithm using the ID-structures of POPELGRAM. The ID-structures implicitly represent the search space. To constraint the space, heuristic information in form of partial feature descriptions is used. This search operation serves as the basis for other operations, to(), e.g. for unifying lexical elements with basic segments. order to facilitate spontaneous speech. The feature set of the head of the associated segment is checked to see if it contains sufficient information for inflecting and linearizing it. For every segment this information has to be specified under the head's feature local. Actual values come either from the lexicon, e.g., gender for nouns, from dependent elements by means of structure sharing, e.g., person and number for a verb from the subject constituent or from a govering constituent, e.g., case for dependent nouns of a verb.</Paragraph>
      <Paragraph position="12"> When all subfeatures of the local feature have values, i.e. a value other than the empty feature set, then the segment is said to be locally complete. The reason for the segment of a DBSoobject not to be locally complete is that DBS--objects that share values with the local feature set are either not yet created or not yet locally complete themselves (e.g., a verb is locally incomplete when a phrase that has to fill the subject role is not yet created), in the first case, the missing objects are requested (using the object's context) from that object of the semantic level above that has created the requesting object, in the second case, DBSobjects that are already connected to the underspecified object are invited to determine the missing values.</Paragraph>
    </Section>
    <Section position="3" start_page="290" end_page="290" type="sub_section">
      <SectionTitle>
4.3 Inflection, Linearization
Reformulation
</SectionTitle>
      <Paragraph position="0"> and Segments from the DBS-level that are locally complete ate inflected and linearized at the ILS-level in POPEL.-HOW. The inflection is performed by means of the package MORPHIX \[Finkler&amp;Neumann88\], which handles most of the inflection phenomena of German. The order of segments in an utterance is syntactically constrained by the LP-part of POPELGRAM. It is assumed that the order of activation of conceptual segments (which is determined using pragmatical knowledge (of.</Paragraph>
      <Paragraph position="1"> \[Reithinger88\])) should be maintained if it is syntactically wellformed; otherwise the segments are reordered by means of relevant LP-structures. Therefore, the order of the input segments affects the variations of the linear order in the resulting utterance, which makes POPEL-HOW more flexible (e.g. by deciding whether to realize an active or passive form). But the order of the input segments can affect lexical choice, too. For example, if the modifier denoting the vehicle (e.g., &amp;quot;airplane&amp;quot;) for a conceptual segment of type &amp;quot;comn-mte-bypublic-transportation&amp;quot; is known in due time the more restricted verb &amp;quot;to fly&amp;quot; is chosen instead of the more general verb &amp;quot;to go&amp;quot;. Otherwise, a 292 5 prepositional phrase realizing the modifier must be verbalized explicitly (e.g., &amp;quot;to go by airplane&amp;quot;).</Paragraph>
      <Paragraph position="2"> It is possible that because of the incremental composition of large structures across several levels new conceptual segments lead to the reformulation of structures at successive levels (cf. \[DeSmedt&amp;Kempen881) as indicated in the following sample utterances: &amp;quot;Mary is going ... uh ... Mary and Peter are going to school.&amp;quot; &amp;quot;Eric goes ... uh ... flies to the USA.&amp;quot; In POPEL-HOW the reformulation of a segment is represented by an object that is in competition to the corresponding already existing object (e.g., the first utterance where the new conceptual segment &amp;quot;Peter&amp;quot; leads to a syntactic segment &amp;quot;Mary and Peter&amp;quot; which is in competition with the noun phrase &amp;quot;Mary&amp;quot;). In order to integrate such new objects the connections to an object which is to be replaced must be broken up and reconnected to the new object (see also fig. 5). Of course, the replacement must be performed for the relevant associated segments, too. For syntactic segnaents we have developed an operation which allows the replacement of parts of a given feature set. In principle this is realized by resetting the corresponding partial feature set (i.e., relevant features get the empty value) and subsequently unifying it with the new information.</Paragraph>
    </Section>
  </Section>
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