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<?xml version="1.0" standalone="yes"?> <Paper uid="J98-3003"> <Title>A Generative Perspective on Verb Alternations</Title> <Section position="5" start_page="403" end_page="409" type="metho"> <SectionTitle> 2 Moens and Steedman (1988) also use this term, but they restrict it to momentaneous events. </SectionTitle> <Paragraph position="0"> Unfortunately, the terminology used in the literature for these kinds of categories varies so much that a standardization seems out of reach.</Paragraph> <Paragraph position="1"> event- 1 fill con~f----~ > 'not-full -state-1 ~ ' '~ ~f~ > pa~>b~tination ~ fill-state-2 ...... ;~.~ ~ water-1 \] value\[ > 'full SitSpec representing a fill-event. Subsumed by the general ontological system, a domain model is defined that holds the concepts relevant for representing situations and that specifies the exact conditions for their well-formedness. We use the term SitSpec for a network of instances of domain model concepts, which will be the input to our generator. The root node of any SitSpec is of the type situation. As an example, the event of a person named Jill filling a tank with water is shown in Figure 2 in a graphical KL-ONE notation (Brachman and Schmolze 1985), with relation names appearing in boxes. The event combines the activity of Jill pouring water into the tank with the fill-state of the tank changing to full. A verbalization of this event can emphasize either of these aspects. Since we decompose event structure in such a way, it follows that the denotations of verbs for verbalizing events need to be fairly complex. The type of event denoted relates to the Aktionsart of the verb: the inherent features characterizing (primarily) the temporal distribution of the event denoted. 3 A generator needs to know these features when verbalizing different kinds of events, so that it can produce (for example) the correct temporal modifier to express the duration of either an activity or a culmination. The variety of phenomena in Aktionsart are far from clear-cut, and there is no generally accepted and well-defined set of features. In the following, we use the terms given by Bussmann (1983) and discuss only those Aktionsart features that are directly relevant for us because they relate types of situations to denotations of verbs. Thus, within the context of our system, we define Aktionsart features in terms of patterns of verb denotations. Table 1 lists the correspondences.</Paragraph> <Paragraph position="2"> Simple cases are stative verbs like to own or to know. Durative verbs characterize continuous occurrences that do not have internal structure, like to sleep, to sit. In the class of nondurative verbs we find the semelfactive ones, which denote a single occurrence, thus in our system a momentaneous activity, as, for example, to knock. Interestingly, an iterative reading can be enforced on a semelfactive verb by a durative adverbial: She poked me for an hour. Transformative verbs involve a change of some state, without a clearly recognizable event that would be responsible for it: The room lit up. The denotation of such verbs thus involves a pre-state and a post-state. In our ontology, these are transitions. Resultative verbs, on the other hand, characterize situations in 3 This is often treated on a par with aspect, but we prefer to make a terminological distinction between the grammaticalized categories such as progressive versus nonprogressive in English (aspeCt),. and the static verb-inherent features.</Paragraph> <Paragraph position="4"> which something is going on and then comes to an end, thereby resulting in some new state (culminations in our ontology). Their denotation includes an activity and a post-state. In the literature, such verbs are often also called inchoative. 4 The final verb-inherent feature we use is the well-known causative, which reflects the presence of a causer in the denotation (as in Figure 2).</Paragraph> <Paragraph position="5"> Verb alternations, as we will discuss them shortly, can involve a shift in Aktionsart and thus a systematic change of the denotation. But first we have to introduce the second major component of verb semantics--sentence meaning.</Paragraph> <Section position="1" start_page="405" end_page="408" type="sub_section"> <SectionTitle> 2.3 Sentence Meaning </SectionTitle> <Paragraph position="0"> A SitSpec representing a possibly complex event structure can be verbalized by a variety of sentences, which can differ in terms of their argument structure, aspectual composition, etc. From the viewpoint of NLG, we wish to select the most appropriate sentence on the grounds of target parameters, such as the salience assignment mentioned in the beginning of the paper. In order to produce a sentence that accomplishes semantic goals of this kind, it is impractical to map the very abstract SitSpec directly to a syntactic structure. Instead, we use a sentence-semantic level of description that allows us, on the one hand, to control those generation decisions that affect the meaning of the sentence, and, on the other hand, to encapsulate the syntactic realization decisions in the front-end generation grammar.</Paragraph> <Paragraph position="1"> 2.3.1 Halliday's Ideational Structure. To describe sentence meaning, we use the &quot;ideational structure&quot; introduced by Halliday (1985). It resembles other approaches based on semantic case roles, but an important feature of Halliday's work is his thorough classification of process types and of the semantic relationships holding between the verb and the other elements in a clause. 5 This extensive analysis renders the approach particularly useful for sentence generation. Halliday's process classification has been further developed for NLG purposes by C. Matthiessen, J. Bateman and others (see, for instance, Matthiessen and Bateman \[1991\]). The resulting &quot;upper model&quot; (UM) is part of the Penman generator and used in our system as well. The UM is a taxonomy of 4 The term inchoative is used to cover a rather broad range of phenomena, including the beginning of an event (e.g., to inflame) or its coming to an end. We think the term is overloaded and prefer to use resultative for the latter group. 5 Halliday proposes two additional levels of sentence description (&quot;metafunctions'), which operate in parallel to ideational structure: the interpersonal and the textual. For our present purposes, we can neglect them; for a broader scope of sentence generation, they are very important. SemSpec and a corresponding sentence.</Paragraph> <Paragraph position="2"> linguistic categories that directs the grammar in verbalizing objects (in the generator's input) in terms of these categories. Hence, the UM can be characterized as mirroring the distinctions made in surface linguistic realizations: Typically, any two distinct UM types correspond to some difference in English sentences.</Paragraph> <Paragraph position="3"> The largest part of the UM is devoted to processes, which are characterized by their verbalization patterns. For our purposes here, we need only a small fragment of the process hierarchy, namely the subtree of material processes. They can be characterized by the fact that English verbalizations of them in present tense typically use the progressive form, as in the house is collapsing (unmarked) as opposed to the house collapses (marked). They typically involve the participant roles &quot;actor&quot; and &quot;actee &quot;6 but differ in terms of constraints on the types of the role fillers, and with respect to their realization in language. Material processes have two subgroups, one of which are nondirected-actions. They do not involve external agency and are mostly intransitive.</Paragraph> <Paragraph position="4"> With such processes, the actee is not a genuine participant, but rather an elaboration of the process. Verbs falling into this category are those of movement, of expressing skills, as well as support verbs like to take as in take a shower. The other subgroup, directed-actions, are always transitive, and they involve an external agent of the process. null The upper model thus reflects the semantic distinctions made by the language, and the systemic-functional grammar takes care of the syntactic realization of these distinctions. Accordingly the lexicalization component we are proposing here is in charge of producing a sentence-semantic specification along the lines of ideational structure (using the upper model categories), such that the relevant decisions affecting sentence meaning can be controlled during lexical choice. As an example, Figure 3 shows one of the SemSpecs and an English sentence that can be derived (as explained in Section 4) from the SitSpec given in Figure 2. Besides actor and actee, the role &quot;destination&quot; is used in the SemSpec; later we will also encounter &quot;source.&quot; The UM is a good starting point, but in some respects the process classification is not quite fine-grained enough. A deficiency that is directly relevant for our treatment of alternations concerns the valency patterns of verbs, where some additional distinctions are needed.</Paragraph> <Paragraph position="5"> 2.3.2 Valency. As introduced by Tesni~re (1959), valency refers to the distinction between actants and circumstantials (central participants associated with the verb versus temporal, locational, and other circumstances). This separation is in principle widely accepted, but views differ on where to draw the line and how to motivate it. The notion of valency was further developed predominantly in German linguistics, with a culmination point being the valency dictionary of German verbs by Helbig and Computational Linguistics Volume 24, Number 3 Schenkel (1973). They made an additional distinction between obligatory and optional actants; Somers (1987, chapter 1) proceeded to propose six different levels of valency binding. He also pointed out that there are different opinions on the type of entities that are subject to a verb's valency requirements: some authors describe them by syntactic class, some by semantic deep cases, and some by their function (subject, object, etc.).</Paragraph> <Paragraph position="6"> Halliday (1985), in his classification, essentially adopts the basic Tesni~rian distinction and suggests some semantic and syntactic criteria for deciding between actants, which he calls participants, and circumstances. Spatio-temporal information, for instance, is generally treated as a circumstance. As a syntactic indicator, for Halliday, participants are typically realized as nominal groups (with some obvious exceptions, as in say that x), and circumstances as prepositional phrases or as adverbs. But neither this syntactic division corresponding to participants and circumstances (direct or indirect object versus adverbs or prepositional phrases), nor the semantic postulate that spatio-temporal aspects are circumstances, holds in general. Regarding spatial relationships, we find verbs that specifically require path expressions, which cannot be treated on a par with circumstances: Consider, for example, to put, which requires a direct object and a destination. Causative to pour requires a direct object as well as a path with either a source, or a destination, or both: pour the water from the can into the bucket. 7 Some verbs, as is well-known, can occur with either a path (Tom walked into the garden) or a place (Tom walked in the garden), and only in the garden can here be treated as a circumstance. And to disconnect requires a direct object (the entity that is disconnected) and a source (the entity that something is disconnected from), which can be omitted if it is obvious from the context: Disconnect the wire! As a step toward a more fine-grained distinction between participants and circumstances, we adopt the three categories proposed by Helbig and Schenkel (1973) and thus distinguish between obligatory and optional participants on the one hand, and circumstances on the other. Moreover, we differentiate between requirements of process types (as encoded in the process taxonomy) and requirements of individual verbs, which are to be encoded in the lexical entries. In a nutshell valency (as a lexical property) supplements the participant/circumstance requirements that can be stated for types of processes.</Paragraph> <Paragraph position="7"> To encode the valency information, we introduce the partial semantic specification (PSemSpec) as one central component of lexical entries. The participant roles stated in the PSemSpec are either obligatory or optional; in the latter case they are marked with angle brackets: to disconnect PSS: (x / directed-action :actor A :actee B < :source C >) With obligatory participants, the verb is only applicable if the elements denoted by these participants are present in the input structure to be verbalized (the SitSpec).</Paragraph> <Paragraph position="8"> Optional participants need not be included in the verbalization: If they are present in the SitSpec, they may be omitted if there is some good reason (e.g., a stylistic preference); if they are not present in the SitSpec, the verb can be used anyway. The disconnect example illustrates that, in contrast to Halliday, we allow for verbs selecting path expressions, here as an optional complement. We can thus distinguish between 7 Given a suitable context, though, the sentence She poured the wine is perfectly acceptable. But this usage seems to be restricted to a small class of digestible liquids.</Paragraph> </Section> <Section position="2" start_page="408" end_page="408" type="sub_section"> <SectionTitle> Stede Verb Alternations </SectionTitle> <Paragraph position="0"> cases like the following: * Tom disconnected the wire {from the plug}. To disconnect requires a source, but it can be omitted in a suitable specific context.</Paragraph> <Paragraph position="1"> * Sally ate. While to eat usually requires a direct object, it can also be used intransitively due to the strong semantic expectation it creates on the nature of the object--independent of the context.</Paragraph> <Paragraph position="2"> * Tom put the book on the table. To put requires a destination, and it cannot be omitted, no matter how specific the context.</Paragraph> <Paragraph position="3"> * The water drained from the tank {in the garage}. Locative circumstances like in the garage are not restricted to particular verbs and can occur in addition to paths required by the verb.</Paragraph> <Paragraph position="4"> The criterion of optionality, as indicated above, singles out the obligatory complements from the other two categories. But how, exactly, can we motivate the distinction between optional participants and circumstances in our framework? By relating the PSemSpec to the SitSpec, via the denotation. In the disconnect case, for instance, the two items connector and connectee are both integral elements of the situation. The situation would not be well-formed with either of them absent, and the domain model encodes this restriction. Therefore, both elements also occur in the denotation of to disconnect, and a coindexed variable provides the link to the PSemSpec. Only when building the sentence SemSpec is it relevant to know that the connectee can be omitted. The connectee in the denotation therefore must have its counterpart in the PSemSpec--that is the source, but there it is marked as optional (see Figure 4 below).</Paragraph> <Paragraph position="5"> With circumstances, the situation is different: A SitSpec is complete and well-formed without the information on, for instance, the location of an event. Hence, a verb's denotation cannot contain that information, and it follows that it is not present in the PSemSpec, either.</Paragraph> </Section> <Section position="3" start_page="408" end_page="409" type="sub_section"> <SectionTitle> 2.4 Lexical Entries </SectionTitle> <Paragraph position="0"> We have introduced the two central components of lexical entries and now give a complete list of the components used in our system. The connotations are not directly relevant for our mechanism of handling verb alternations, therefore they will not be dealt with here. Salience assignment will be discussed in Section 4.</Paragraph> <Paragraph position="1"> Denotation: A partial SitSpec that defines the applicability condition of the lexeme: If its denotation subsumes some part of the input SitSpec, then (and only then) it is a candidate lexical option for the verbalization.</Paragraph> <Paragraph position="2"> Covering: The subset of the denotation nodes that are actually expressed by the lexeme. One of the constraints for sentence production is that every node be covered by some lexeme.</Paragraph> <Paragraph position="3"> Partial SemSpec (PSemSpec): The contribution that the lexeme can make to a sentence SemSpec. By means of shared variables, the partial SemSpec is linked to the denotation.</Paragraph> <Paragraph position="4"> Connotations: Stylistic features pertaining to formality, floridity, etc.</Paragraph> <Paragraph position="5"> Salience assignment (for verbs only): A specification of the different degrees of prominence that the verb assigns to the participants.</Paragraph> <Paragraph position="6"> Excerpts from sample lexical entries for verbs.</Paragraph> <Paragraph position="7"> Alternation rules: (for verbs only): Pointers to lexical rules that represent alternations the verb can undergo (see Section 3).</Paragraph> <Paragraph position="8"> Morphosyntactic features: Standard features needed by the surface generator to produce correct utterances.</Paragraph> <Paragraph position="9"> Figure 4 gives excerpts from sample lexical entries, which demonstrate the linking between entities from the denotation and the PSemSpec. Notice that the linking is shown for the base form of the verb, which can be quite simple, as in open or move. Items appearing with an asterisk in front of them are optional in the SitSpec: for example, a SitSpec underlying an open-event is well-formed without a causer being present. These items get verbalized with the help of rules such as the alternation rules to which we turn in the next section. In the lexical entries, the names of applicable alternation rules are listed below the line.</Paragraph> </Section> </Section> <Section position="6" start_page="409" end_page="423" type="metho"> <SectionTitle> 3. Alternations </SectionTitle> <Paragraph position="0"> Having explained denotations and PSemSpecs, specifically for verbs, we can now turn to the task of accounting for the different alternations a verb can undergo. Under this heading, we will look both at the so-called transitivity alternations, which are characterized by a change in the number of participants (e.g., the causative), and at diatheses,</Paragraph> <Section position="1" start_page="410" end_page="410" type="sub_section"> <SectionTitle> Stede Verb Alternations </SectionTitle> <Paragraph position="0"> which only affect the mapping between the participants and syntactic realization (e.g., the passive). Thus, a variant such as topicalization does not qualify as an alternation, since the syntactic realization of the participants remains unchanged; they are merely reordered. The most comprehensive source of information on alternations is the compilation by Levin (1993); we will now look at some of the more prominent alternations listed there and characterize them in terms of changes in denotation and valency of the verbs.</Paragraph> </Section> <Section position="2" start_page="410" end_page="412" type="sub_section"> <SectionTitle> 3.1 Alternations as Meaning Extensions </SectionTitle> <Paragraph position="0"> A simple way of treating alternations is to use a separate lexical entry for every configuration, but that would clearly miss the linguistic generalizations. Instead, we wish to represent the common &quot;kernel&quot; of the different configurations only once, and use a set of lexical rules to derive the alternation possibilities. Jackendoff (1990) is concerned with this problem for a number of alternations; specifically, in his framework of lexical-conceptual structure (LCS) he seeks to explain the relationships between stafive, inchoative, and causative readings of a verb. In Jackendoff's analysis, the forms are derived sequentially by embedding in the primitives INCH and CAUSE, respectively: null * stative: BE(\[Thi,x \](A), \[INa \[Thing \]A \]) * inchoative: INCH \[BE(\[Thing \](A), \[INd \[Thing \]A \])\] * causative: CAUSE(\[Thi,g \]A, INCH \[BE(\[Thing \](A), \[INd \[Thing \]A \])\] For our NLG purposes, the idea of deriving complex verb configurations from more basic ones is attractive, but it is necessary that we relate verb meaning to our explicit treatment of event structure, instead of masking that structure with a primitive such as INCH. When verbalizing a SitSpec, we first have to determine candidate lexemes, i.e., match the SitSpec against lexicon entries; having only one lexical entry for a verb reduces the search space considerably. Moreover, since the verb entry will be the most basic form, its denotation is relatively simple and therefore the matching is inexpensive. Finding more complex verb configurations will then require some further matching, but only locally and to those verbs that have already been determined as verbalization options.</Paragraph> <Paragraph position="1"> In general the idea is to see verb alternations not just as relations between different verb forms, but to add directionality to the concept of alternation and treat them as functions that map one into another. From this viewpoint, there are two groups of alternations: (1) Alternations that do not affect the denotation of the verb. Examples are the passive or the substance-source alternation (The tank leaked oil; Oil leaked from the tank): The truth conditions do not change. (2) Alternations that do change the denotation of the verb.</Paragraph> <Paragraph position="2"> The second group is the critical one, because if we derive verb configurations from others and rewrite the denotation in this process, it has to be ensured that the process is monotonic. 8 Therefore we define directionality for group (2) to the effect that an alternation always adds meaning: The newly derived form communicates more than the old form the denotation gets extended. This notion is different from the standard, Computational Linguistics Volume 24, Number 3 of extension rules for which we can give a clear definition in terms of Aktionsart features, as they were introduced in Section 2.2. These rules extend the denotation of a verb and rewrite its PSemSpec in parallel to reflect the change in valency; the result is a new verbalization option, which can differ from the previous one in terms of coverage or attribution of salience (see Section 4). The rules will be conveniently simple to state, thanks to the upper model, which provides the right level of abstraction from syntax.</Paragraph> <Paragraph position="3"> To illustrate the goal we return to the example of Tom removing the oil from an engine. If a SitSpec encodes this situation, then to drain is a candidate lexeme. While it can appear in a number of different configurations, we wish to match only one of its forms against the SitSpec, though. This is the most basic one, denoting an activity: The oil drained from the engine. Here, the case frame of the verb has to encode that from the engine is an optional constituent. Now, an extension rule has to systematically derive the causative form: Tom drained the oil from the engine. And also from the first configuration, another rule derives the resultative reading, which adds the information that the engine ended up empty: The engine drained of the oil. Here, of the oil is an optional constituent. To this last form, a causative extension can apply and yield Tom drained the engine of the oil.</Paragraph> <Paragraph position="4"> To compute these configurations automatically, such that valency and meaning are changed in parallel, we define an alternation or extension rule as a 5-tuple with the following components: NAM: a unique name; DXT: extension of denotation; C0V: additions to the covering-list; ROC: role changes in PSemSpec; NR0: additional PSemSpec roles and fillers.</Paragraph> <Paragraph position="5"> The DXT contains the denotation subgraph that the new verbalization has in addition to the old one. The syntax is, of course, the same as that of the denotation of a lexical entry. Specifically, it can contain variables; these can co-occur in the C0V list: the items that the new verbalization covers appear in addition to those of the old one. R0C is a list of pairs that exchange participant role names or the UM type in the PSemSpec; this replacement can also change optionality. For example, (< :actee > :actor) means &quot;replace the term :actee in the PSemSpec of the old verbalization, where it was optional, with :actor, which is not optional.&quot; Finally, NR0 contains new roles and fillers that are to be added to the new PSemSpec; these will also contain variables from the denotation extension.</Paragraph> <Paragraph position="6"> Applying such a rule to a verbalization option vo works as follows: Add the contents of DXT to the denotation of vo, and match the new part against the SitSpec. If it matches, make a copy vo t of vo and assign it a new name as well as the denotation just formed. Add the C0V list, which has been instantiated by the matching, to the covering-list of vo'. Exchange the role names in the PSemSpec of vo ~ as prescribed by ROC, and, importantly, in the order they appear there. Finally, add NR0 to the PSemSpec. In the following, we first give an example for a rule that changes only the PSemSpec without affecting the denotation. Afterwards, we describe those alternations that change the Aktionsart of the verb and thus the form of the SitSpec expressed (as discussed in Section 2.2): the stative-resultative, causative, and locative alternations. Before introducing these rules, it should be emphasized that we do not provide applicability conditions for the alternation and extension rules, which would inspect some verb denotation and on that basis decide whether an alternation can apply; instead, the rules are triggered directly from the lexical entry of a verb. Whether general applicability conditions can be specified, so that the rules need not be attached to each individual verb, is a central open research question that linguistic alternation research is concerned with.</Paragraph> </Section> <Section position="3" start_page="412" end_page="416" type="sub_section"> <SectionTitle> 3.2 Encoding Alternation Rules </SectionTitle> <Paragraph position="0"> The best-known alternation that affects only the valency of the verb is the passive, which we do not investigate here. Instead, we show one alternation that is particularly relevant for verbs in the domain of substances and containers.</Paragraph> <Paragraph position="1"> Substance-Source Alternation. Example: The tank leaked water / Water leaked from the tank. This is an alternation discussed by Levin (1993); to make use of it here, we have to add directionality and declare one of the two configurations as more basic. Levin lists verbs of &quot;substance emission&quot; as undergoing it, for example drip, radiate, sweat, and leak. 9 To decide on the more basic form, we use the fact that in The tank leaked water the water is an optional constituent, and hence the minimal configuration of the verb is The tank leaked. With the from configuration, no deletion is possible.</Paragraph> <Paragraph position="2"> As a representative of the verb class, we show the denotation and PSemSpec of to Let us now consider several alternations that change denotation, and hence are extensions. null Stative-Resultative. Example: Water filled the tank / The tank filled with water. In discussing verbs that denote a state, Jackendoff (1990) points out that fill, cover, surround, and saturate can describe either a state or an inchoative event, and encodes the difference with the primitive INCH we have shown in the introduction to this section. Our goal is to do without the primitive, and to define the change in terms of the Aktionsart of the verb; to this end, we use resultative in place of inchoative (see Section 2.2).</Paragraph> <Paragraph position="3"> 9 Unnoticed by Levin, to leak can also be a verb of substance &quot;intrusion,&quot; as in The camera leaked light. This reading, which we do not handle here, reverses the directionality of the path involved.</Paragraph> <Paragraph position="4"> Computational Linguistics Volume 24, Number 3 On a similar matter, Levin (1993) describes the &quot;locatum subject&quot; alternation; which for instance holds between I filled the pail with water and Water filled the pail. It thus relates a causative and a noncausative form. Levin states that the alternation applies to a class of &quot;fill verbs,&quot; of which there are many more than the four given by Jackendoff. Her alternation is not exactly the one we need here, since it also involves a causative form; deriving the causative is a separate step in our framework.</Paragraph> <Paragraph position="5"> What we need here is a mixture of Jackendoff's and Levin's insights: Several of Levin's fill verbs can be both transitive and intransitive, and some of the intransitive readings denote 'to become Xed'. Among these verbs are fill, flood, soak, encrust, or saturate: The kitchen flooded with water means the same as The kitchen became flooded with water. For this subgroup of the fill verbs, we define an extension rule that derives a resultative reading from a state reading. 1deg Notice that this is different from Levin's :actor B :actee A < :destination C >) When matching it against a SitSpec with a tank and water, this yields the verbalization The water filled the tank, covering only the post-state of the SitSpec. Now, the alternation rule extends the denotation to also cover the event and the activity that brings the filling about. Applying the changes to the PSemSpec results in (X / nondirected-action :lex fill :inclusive B :actor A < :destination C >) which corresponds to the sentence The tank filled with the water.</Paragraph> <Paragraph position="6"> A few stative verbs cannot be resultative without being also causative. Consider to cover in these examples from Jackendoff: Snow covered the ground.</Paragraph> <Paragraph position="7"> ,The ground covered with snow.</Paragraph> <Paragraph position="8"> Bill covered the ground with snow.</Paragraph> <Paragraph position="9"> 10 The two roles &quot;inclusive&quot; and &quot;of-matter&quot; (used later) are the roles used by Penman to realize the desired structure, but they are not very good descriptions of these semantic relationships. For a more systematic treatment, for instance along the lines of Somers (1987), the upper model needs to be extended. See Section 4.1.</Paragraph> <Paragraph position="10"> For these, a stative-culmination extension derives the resultative + causative form directly from the stative one. The rule is similar to the one given above, so we do not show it here.</Paragraph> <Paragraph position="11"> Causative Extensions. Example: The napkin soaked ! Tom soaked the napkin. Levin discusses a causative/inchoative alternation that applies to a large number of verbs. The class formed by them is somewhat heterogeneous with respect to Aktionsart, though; it contains, for example, to turn as well as to open. The former is in its basic form durative (The wheels turned), and the latter transformative (The door opened). Accordingly, we split the alternation in two, which only differ in the DXT component, reflecting the difference in Aktionsart. The durative-causative extension adds a causer to the denotation and makes the former :actor the new :actee. It equally applies to semelfactive verbs denoting a momentaneous activity: The bell rang / The visitor rang the bell. The resultativecausative extension also covers the activity, because Tom opened the door expresses that Tom did something to achieve the change of state. The causer itself is not covered though, because it still has to be verbalized separately.</Paragraph> <Paragraph position="12"> second Tom closed thedoor ~omThe doorclosed.</Paragraph> <Paragraph position="13"> Locative Extensions. Example: (a) Sally sprayed the wall with paint. / (b ) Sally sprayed paint onto the wall. The locative alternation has been studied by lexical-semanticists extensively. Its characteristic is that configuration (a) of the verb conveys that something is performed in a &quot;complete&quot; or &quot;holistic&quot; manner, whereas configuration (b) lacks this facet of meaning. Levin points out that this alternation has received much attention in linguistics research and notes that, in spite of the efforts, a satisfactory definition of the holistic facet has not been found. Jackendoff, in his treatment of the alternation, suggests encoding the holistic feature in a primitive: The function ONd is a derivative of ON and means that something &quot;distributively&quot; covers a surface, e.g., the paint covers all of the wall. Introducing a primitive, though, amounts to conceding that no explanation in terms that are already known can be given. We cannot solve the question of &quot;holisticness,&quot; either, but we want to point to the fact that the two verb configurations correlate with a change in Aktionsart: Sally sprayed paint onto the wall is durative (she can do it for two hours), whereas Sally sprayed the wall with paint is transformative (she can do it in two hours). That observation leads us to propose that the example is best analyzed as involving a mere activity in the with configuration, and an additional transition in the onto configuration. Support for this analysis comes from Pinker (1989), who postulates a change in meaning when moving from one configuration to the other: In (b) above, Sally causes the paint to move onto the wall whereas in (a), Sally causes the wall to change its state by means of moving the paint onto it. the alternation, that the motion (here: spray) causes an effect on the surface/container. While we decided not to discuss applicability conditions here, we support the idea that the difference between (a) and (b) can be expressed with an additional change of state. In our framework, we thus assign two different SitSpecs to the sentences, one activity and one event, as shown in Figure 5.</Paragraph> <Paragraph position="14"> The crucial point now is that the first SitSpec is fully embedded in the second; this is in correspondence with the truth conditions: If Sally has sprayed the wall with paint, then she also has sprayed paint onto the wall. To generalize the correspondence to an extension rule, we need to assume in the domain model a concept like completion-state, which is to subsume all those states in the domain model that have &quot;extreme&quot; values: an empty bucket, a fully loaded truck a completely covered surface, and so forth. The exact interpretation of completion-state is the open question that Levin (1993) referred to, and that Jackendoff treated with his d subscript. We do think, though, that an abstract state in the domain model, which subsumes a range of the concrete states, is preferable to introducing a primitive on the linguistic level (unless the primitive is relevant for other linguistic phenomena as well).</Paragraph> <Paragraph position="15"> The following alternation rule applies to durative verb readings that denote activities of something being moved to somewhere, and extends them to also cover the post-state, which must be subsumed by completion-state. In this wan it derives reading (a) from (b) in the spray example, and analogously for the other verbs undergoing the alternation, e.g.: Tom loaded hay onto the wagon ! Torn loaded the wagon with hay; Jill stuffed the feathers into the cushion ! Jill stuffed the cushion with the feathers. The PSemSpec is modified as follows: The former :destination (wall) becomes the new :actee, whereas the former :actee (paint) now fills the role < :inclusive >, and is optional there, because Jill stuffed the cushion is also well formed.</Paragraph> </Section> <Section position="4" start_page="416" end_page="416" type="sub_section"> <SectionTitle> Stede Verb Alternations </SectionTitle> <Paragraph position="0"> Levin distinguishes two kinds of locative alternation: the spray/load alternation just discussed and the clear (transitive) alternation. The latter applies only to the verbs clear, clean, drain, empty and can be seen as the &quot;semantic inverse&quot; of the spray/load alternation, because one group of verbs denotes activities of placing something somewhere, and the other describes activities of removing something from somewhere; but both have the same holistic effect in one of the verb configurations. Thus, the rule for the clear-alternation is very similar to the one just shown. It derives, for example, Tom drained the container of the water from Tom drained the water from the container, n The clear verbs, except for to clean, can in addition be intransitive, and Levin states a separate alternation for them. For to drain, the first configuration is The water drained from the tank, and the second is either The tank drained or ?The tank drained of the water. According to Levin, &quot;the intransitive form may be best in the absence of the of-phrase&quot; (Levin 1993, 55). The SitSpec denoted by the first configuration is: The water drained from the tank.</Paragraph> <Paragraph position="2"> Note that our durative-causative extension rule given above applies in this case and extends the coverage of the SitSpec to one corresponding to Tom drained the water from the tank. A rule that is parallel to that for the transitive case is given below; it derives : of-matter > is optional, we can also produce ?ThetankdrainedofthewateG sincethe <</Paragraph> </Section> <Section position="5" start_page="416" end_page="418" type="sub_section"> <SectionTitle> 3.3 Deriving Alternations Successively </SectionTitle> <Paragraph position="0"> The extension rules, as we have introduced them above, constitute a framework for systematically deriving more complex verb configurations from simpler ones; the output produced by one rule serves as input to another. Figure 6 provides a synopsis: The 11 We ignore the role of the definite determiner here, which in fact has critical influence on the holistic interpretation of mass nouns. See, for example, White (1994).</Paragraph> <Paragraph position="2"> Dependency of extension rules.</Paragraph> <Paragraph position="3"> boxes contain the denotation patterns that correspond to the Aktionsart feature, and the rules transform a configuration with one Aktionsart into another. In this graph, every verb base form has an entry point corresponding to the Aktionsart of its most basic configuration. Examples: to fill is stative, to drain is durative, to open is transformative, to remove is resultative + causative. The &quot;double box&quot; in the middle is the entry point for both transformative and resultative verbs, but the incoming arrows produce resultative forms. From the entry point of a verb, arcs can be followed and rules applied if the respective alternation is specified in the lexical entry. The six categories account for the Aktionsart features listed in Section 2.2, and the rules take care of possible shifts between them. Thus, the full range of SitSpecs that our ontology allows is being covered.</Paragraph> <Paragraph position="4"> To illustrate the functionality, we return to the example of to drain. Figure 7 shows how the extension rules successively derive the various configurations. Apart from the passive, this is the complete &quot;alternation space&quot; of to drain according to Levin's (1993) catalogue. Notice that the examples given also cover the four different drain clauses needed to produce the alternative sentences given in (1) in the introduction.</Paragraph> <Paragraph position="5"> 4. Implementation: Two-Step Sentence Generation with MOOSE The MOOSE sentence generator grew out of experiences with building the TECHDOC system (RSsner and Stede 1994), which produces instructional text in multiple languages from a common representation. Specifically, MOOSE accounts for the fact that events can receive different verbalizations even in closely related languages such as English and German. It is designed as a sentence generation module that pays attention to language-specific lexical idiosyncrasies, and that can be incorporated into Derivation of drain configurations.</Paragraph> <Paragraph position="6"> a larger-scale text generator} 2 In the following, we first describe the overall system architecture, then discuss the process of lexicalization in some detail, and finally turn to the selection of a verb alternation on the basis of salience parameters.</Paragraph> </Section> <Section position="6" start_page="418" end_page="420" type="sub_section"> <SectionTitle> 4.1 System Architecture </SectionTitle> <Paragraph position="0"> Figure 8 provides an overview of the architecture of MOOSE. The generator assumes a language-neutral level of event representation, the situation specification SitSpec (see the example in Figure 2). The SitSpec instantiates concepts from a domain model, which is implemented in the KL-ONE language LOOM (MacGregor and Bates 1987).</Paragraph> <Paragraph position="1"> Using the denotations of the lexicon entries of the target language, the lexical options for verbalizing the SitSpec are determined. In the next step, for verbs, the applicable alternations and extensions are computed and added to the set of options. Then a language-specific semantic specification SemSpec (see the example in Figure 3) is constructed in accordance with generation parameters pertaining to brevity, salience, and stylistic features. The SemSpec is then handed over to a surface generator: Penman (Penman group 1989) for English, and a variant developed at FAW Ulm for German.</Paragraph> <Paragraph position="2"> The SemSpec language is a subset of the input representation language that was developed for Penman, the sentence plan language (SPL) (Kasper 1989). An SPL expression consists of variables, types, and case roles; an example was given in Figure 3.</Paragraph> <Paragraph position="3"> Penman and SPL are based on the upper model (UM) (Bateman et al. 1990) introduced 12 Fr6hlich and van de Riet (1997) describe how MOOSE is employed in the generation component of an information system.</Paragraph> <Paragraph position="4"> in Section 2.3.1. For any type appearing in an SPL, Penman needs to know by which UM type it is subsumed, so that appropriate generation decisions can be made.</Paragraph> <Paragraph position="5"> The way we use Penman and the UM in the Moose architecture is somewhat different from the original Penman conception. In Penman, the domain model was supposed to be subsumed by the UM, which indeed simplifies generation from input that uses domain model concepts. However, the range of alternative verbalizations</Paragraph> </Section> <Section position="7" start_page="420" end_page="420" type="sub_section"> <SectionTitle> Stede Verb Alternations </SectionTitle> <Paragraph position="0"> that can be produce d from the same input is seriously limited under this approach (see Stede and Grote \[1995\]), and therefore Moose opts for a complete separation of DM and UM; they are distinct taxonomies. Consequently, as opposed to a general SPL term used in Penman, a SemSpec used in Moose must contain only upper model concepts and no domain model concepts.</Paragraph> <Paragraph position="1"> Furthermore, since our system takes lexicalization as the decisive task in mapping a SitSpec to a SemSpec, the UM concepts referred to in a SemSpec must be annotated with :lex expressions. Thus, a SemSpec is a lexicalized structure, and accordingly, Moose interprets the upper model as a taxonomy of lexical classes. This contradicts the Penman philosophy of viewing the UM as abstract semantics and clearly distinct from the generation grammar, which in accordance with systemic-functional linguistics is an integrated lexicogrammar, with &quot;lexis as most delicate grammar&quot; (Hasan 1987). This idea, however, has been a theoretical rather than a practical one, and lexical matters thus have not been a strong point of Penman. For instance, the distinction between obligatory and optional participants of a verb was quite blurred. Also, Penman allowed only for very simple lexical choice mechanisms, as it assumed a straightforward one-to-one mapping between concepts and words. MOOSE overcomes these problems by assigning a central role to the lexicon, placing a lot of information in it, and taking it as the crucial device for the SitSpec-SemSpec mapping. SemSpec, then, is an intermediate level of representation that reflects sentence semantics and that mediates between the language-neutral conceptual representation and linguistic realization. The simple form of our alternation rules shown in the last section, which abstract over syntactic realization, demonstrates the utility of SemSpec as a level of description.</Paragraph> <Paragraph position="2"> In practice, our aim to upgrade SPL from a convenient input notation of a front-end NLG module to a systematic and well-motivated level of description involves not only building Moose &quot;around&quot; Penman, but also making some changes to the upper model and the generation grammar. But for the purposes of this paper, which focuses on the semantics of verb alternations and their role in NLG, we avoid dealing with Penman's internals and rather treat it as a &quot;black box.&quot;</Paragraph> </Section> <Section position="8" start_page="420" end_page="421" type="sub_section"> <SectionTitle> 4.2 Lexicalization </SectionTitle> <Paragraph position="0"> In order for serious lexical choices to be possible, the first step of lexicalization in Moose consists of determining the set of verbalization options: all the lexemes whose denotations can potentially cover some part of the input SitSpec. Since we represent the internal structure of events, the denotation of a lexeme need not be a single concept; instead, it can be a complete configuration of concepts and roles. The consequences are a higher computational cost in finding lexical options, but also a greater flexibility in finding different verbalizations of the same event. As an example, consider the denotation of the causative reading of tofilh</Paragraph> <Paragraph position="2"> Given some input SitSpec involving filling, the variables of the denotation are bound to instances or atomic values of the SitSpec when it is matched against the denotation.</Paragraph> <Paragraph position="3"> The filler of the value role in the post-state appears in angle brackets because it is a default value. The accompanying partial SemSpec of to fill contains the same variables: Computational Linguistics Volume 24, Number 3 (x / directed-action :lex fill :actor B :actee A :inclusive C <:destination D>) When the denotation is matched against a SitSpec, the variable bindings are propagated to the partial SemSpec; and when it is later unified with the partial SemSpecs corresponding to the other elements, a complete SemSpec results, from which Penman produces a sentence like Jill filled the tank with oil. (If the value is different from 'full, it also gets verbalized, such as in Jill fflled the tank to the second mark.) Importantly, the matching between denotations and SitSpec does not test for identity, but for subsumption--it exploits the functionality provided by LOOM. In this way, the selectional restrictions of verbs are checked when the lexical options are determined. Moreover, the matcher finds not only the most specific word, but also the applicable more general candidates, which is helpful, for instance, in achieving stylistic effects, and in avoiding undue repetitions of the same specific term.</Paragraph> <Paragraph position="4"> Since we are using relatively fine-grained representations for SitSpecs and denotations, the generation of variants in incorporation is enabled by the covering mechanism in conjunction with the subsumption check. In the example go by plane/fly, the general verb to go covers only the move concept, and the role instrument-plane is left to be expressed by a prepositional phrase; whereas the specific verb toffy covers the whole configuration. In this fashion, quite different coverings of the input SitSpec are possible; for instance, MOOSE produces Tom poured water into the tank until it was full and Tom filled the tank with water (amongst others) as paraphrases of the same event.</Paragraph> <Paragraph position="5"> After the initial matching between denotations and SitSpec, the various alternations are computed for those verbs whose base form has been found as a candidate (step 2 in Figure 8). Their lexical entry specifies which alternation/extension rules apply, and they are executed sequentially, as outlined in the previous section, and demonstrated for to drain in Figure 7. For any extension rule that adds new items to the denotation, the new material is matched against the SitSpec to ensure that the alternation is applicable, and to compute the additional covering. In this way, all the applicable alternated forms of a verb are added to the pool of verbalization options.</Paragraph> <Paragraph position="6"> The set of all lexemes that successfully matched some part of the SitSpec, together with the alternated verb forms, constitute the search space for constructing an appropriate SemSpec. The options are first brought into an order of preference (step 3 in Figure 8) according to various parameters such as the desired salience assignment, which is explained in the next section. Considering the options in this order, a complete and well-formed SemSpec is built from the partial SemSpecs that are associated with some of the lexical options--those that collectively cover the entire SitSpec and thus will take part in the sentence. This is done by a unification process driven by the candidate verb options; recall that their PSemSpec consists of an upper model process and the mappings from situation elements to process participants, which is achieved by co-indexing with positions in the denotation. By means of sharing this information between denotation and PSemSpec, the lexicon entries serve as a &quot;bridge&quot; between the SitSpec to be verbalized and the intermediate representation SemSpec.</Paragraph> <Paragraph position="7"> For more details on the kinds of mono- and multilingual variation produced by MOOSE, and on the lexicalization algorithm, see Stede (1996b).</Paragraph> </Section> <Section position="9" start_page="421" end_page="422" type="sub_section"> <SectionTitle> 4.3 Producing Salience Variation with Alternations </SectionTitle> <Paragraph position="0"> Having explained the basic machinery of MOOSE, we now demonstrate how the generator can make an informed choice among the set of possible verb alternations on the basis of a salience parameter, Since a full-fledged treatment of the role of salience</Paragraph> </Section> <Section position="10" start_page="422" end_page="423" type="sub_section"> <SectionTitle> Stede Verb Alternations </SectionTitle> <Paragraph position="0"> is well beyond the scope of this paper, 13 we will merely sketch a possible division of labor between text planning and sentence planning, and then describe the role of verb alternations as one means of realizing generation goals related to salience. Specifically, we show under what circumstances the various drain sentences given in the examples in (1) in Section 1 (and in Figure 7) are produced by MoosE.</Paragraph> <Paragraph position="1"> When text planning has been completed and linearization as well as the &quot;chunking&quot; of the material into sentence-size pieces is accomplished, Moose takes over to perform the necessary sentence planning, which includes lexicalization. Thus, text planning has produced a sequence of SitSpecs, which may be enriched with information pertaining to the relative salience of the elements. This information can result from constraints on theme development, from rhetorical strategies, or from other considerations at the discourse level. For instance, in the sample texts given in (1) at the beginning of Section 1, different theme developments are responsible for the different usages of to drain.</Paragraph> <Paragraph position="2"> For sentence planning, we assume that individual nodes of a SitSpec can have a foreground, background, or optional label attached to them (but they need not). Then, a realization is to be found that signals the differences in prominence on the linguistic surface. In general, there is no one-to-one correspondence between the configuration of salience labels and linguistic realization, though. Instead, we view salience goals as goals that the generator tries to fulfill if possible, similar to certain stylistic goals (see Stede \[1996a\]). Thus, generation becomes a matter of constraints (say the right thing) and preferences (try to say it in a particular way), similar to Hovy's (1988) distinction between &quot;prescriptive&quot; and &quot;restrictive&quot; planning.</Paragraph> <Paragraph position="3"> What, then, is the role of verb alternations in assigning different degrees of salience? Talmy (1988) listed a number of morphological and syntactic means to distribute salience across the elements of a clause. For instance, he suggested the hierarchy sub-ject > direct object > indirect object > oblique, ranging from the most salient to the least salient. From a slightly different perspective, Kunze (1991) was concerned with differences in salience between similar verbs. He advanced the view that they share a common underlying base form and differ, inter alia, in distributing salience via their case roles. For our purposes here, we can adapt these insights (with some simplification) and state that an element is placed in the foreground if it is mapped to the role actor (best) or to actee (second best). Correspondingly, it is placed in the background if it corresponds to a circumstance, i.e., a role that is not part of the verb's case frame.</Paragraph> <Paragraph position="4"> Now, consider again the sentences in Figure 7. On the one hand, (0) and (1) omit the fact of Tom's causing the event, and hence are preferred only if the respective SitSpec node is labeled as optional. On the other hand, (0, 2) and (1, 3) differ in that the former render the oil prominent, while the latter emphasize the engine. Figure 9 shows the common SitSpec underlying the four sentences, and a set of salience labels attached to three nodes, where the numbers correspond to the target sentences. For example, when sentence (1) is the preferred output, the SitSpec would have an opt label at node tom-1 and an fg label at node engine-1.</Paragraph> <Paragraph position="5"> For any verbalization option, base forms and alternations alike, the number of fulfilled salience goals can be computed straightforwardly: Since variables in denotations and PSemSpecs are co-indexed, we can determine for every salience label in the SitSpec how the corresponding element participates in the SemSpec. Using the criteria given above, preference values result for the various options, and they are factored into the overall preference ranking of the verbalization options. All other things being 13 Pattabhiraman (1992) devoted a dissertation to the topic of salience in NLG.</Paragraph> <Paragraph position="6"> SitSpec representing a drain event.</Paragraph> <Paragraph position="7"> equal, the verb alternation that accomplishes the best realization of the salience labels outranks the other options and thus gets selected for building the SemSpec. Again, notice that other syntactic and morphological means (e.g., expressing an element with a separate word versus incorporating it into another word) for assigning salience can be integrated into this scheme.</Paragraph> </Section> </Section> class="xml-element"></Paper>