



























                              Research Report AI-1989-04


                           Artificial Intelligence Programs

                              The University of Georgia

                                Athens, Georgia 30602



                                Available by ftp from

                                  aisun1.ai.uga.edu

                                    (128.192.12.9)



                                   Series editor: 

                                  Michael Covington

                              mcovingt@aisun1.ai.uga.edu









                 PROBLEMS IN APPLYING DISCOURSE REPRESENTATION THEORY


                                   William H. Smith



                                   Piedmont College

                                  Demorest, GA 30535


                           Artificial Intelligence Programs


                              The University of Georgia

                                Athens, Georgia 30602


          Introduction



               Discourse Representation Theory (DRT) was developed by Hans

          Kamp 1981 in order to combine "a definition of truth with a

          systematic account of semantic representations (277)".  The


          semantic representations produced are to provide a bridge between

          syntactic parses and model theoretic semantics such that the

          representations can be used to determine the truth conditions of

          a discourse.  This report describes that theory, both the


          original, basic form and some extensions that have been suggested

          by Kamp and others, and applies it to a "real" discourse in order

          to indicate further extensions that will be necessary if DRT is

          to be used as a complete theory of semantic representations.


               Truth in model theoretic semantics is determined by a

          mapping from a representation of the discourse to a model, a

          mapping that preserves the properties and relationships of the

          discourse.  A model consists of two sets: a set of entities (the


          universe) and a set of properties of those entities and relations

          that hold among them.  The discourse representation must likewise

          consist of two sets, a set of referenced items and a set of

          propositions about those items.  A discourse is held to be true


          in a model if there is a mapping such that the set of referenced

          items maps to a subset of the universe and each property or

          relation expressed by the propositions is true of the

          corresponding entities in the model.  


               Kamp addressed the questions of how a discourse places items

          in the set of referenced items and of how anaphoric relations can

          be expressed in the discourse representation.  In particular, he

          was concerned with situations in which an item should not be


          placed in that set yet should serve as the antecedent for an

          anaphoric relationship (the so-called "donkey sentences").  To

          that end, he developed the basic version of DRT (1981).  Part 1









                                                                          2


          of this report describes that basic theory.  Part 2 presents some

          extensions to the basic theory.  Part 3 describes an attempt to

          represent a text in DRT and suggests further extensions that

          might be made.



          1.   Discourse Representation Theory


               The central notion of DRT is the Discourse Representation


          Structure (DRS).  A DRS K is a pair <U, C>, where U is a set of

          reference markers (the universe) and C is a set of conditions

          (properties, relations, or complex conditions--negation,

          disjunction, or implication).  The initial DRS, K0, contains none


          of the information in the discourse. As the discourse is

          processed, the DRS construction algorithm produces a series of K'

          as it incorporates material from the discourse into K.  For

          example 



          (1)       Pedro owns a donkey.

                    K:<U:{R1, R2},

                       C:{Pedro(R1),


                          donkey(R2),

                          own(R1, R2)}>


               Although U and C are described as sets, at least one of


          these must be ordered by time of introduction into the discourse

          if the construction algorithm is to work properly in assigning

          antecedents to anaphoric expressions (cf Goodman 1988). 

          Antecedent assignment is accomplished by finding an item in U


          that agrees with the anaphoric expression (for pronouns, an

          entity that agrees in gender and number). Thus, (2) shows an

          extension of (1) (:= is an assignment operator, as in Pascal; +

          is union of sets):



          (2)       He beats it.

                    K: <U, 

                        C := C + {beat(R1, R2)}>



          (The basic theory would add R3, R4 to U, and then set them equal

          to R1, R2, respectively.  A later "clean-up" operation would

          eliminate these redundant discourse markers.  Here we assume that



          









                                                                          3






          the clean-up operation has been applied.)  A discourse of any

          size, however, is likely to include several such entities; most

          often, the conflict is resolved by selecting from the candidates

          the one that was most recently encountered.  In order to make


          that selection possible, the set of items must be ordered, and

          must in fact be re-ordered every time reference is made to an

          entity.

               For purposes of exposition, K0 is usually treated as


          consisting of empty sets.  Such is often not the case in real

          discourse, where referring items are often exophoric, their

          referents to be found in the nonlinguistic context or in the

          shared knowledge of the participants.  While DRT allows for such


          references, it is not clear how or when the antecedents are to be

          entered into K.

               Kamp 1985 describes the DRS construction algorithm is "a set

          of rules that operate, in a roughly top-down manner, on the nodes


          of the parse tree, (2)" converting those nodes into the

          conditions of C and, when appropriate, introducing new reference

          markers into U.  As was noted in the introduction to this report,

          the basic version of DRT is directed toward the role of noun-


          phrase (NP) nodes in the discourse--their relationship to U.

               It would seem, at first glance, that every NP should be

          associated with an entity in the model and should therefore have

          a corresponding marker in U.  (That view is, of course, a great


          oversimplification.  Most work in DRT has limited itself to

          singular concrete NPs, where the oversimplification is not so

          drastic.)  When the algorithm encounters a NP, it should either

          associate it with a marker already present in U (anaphora) or


          introduce a new marker, and these were the problems that Kamp

          1981 addressed.

               The main problem for anaphora is that theories of sentential

          syntax do not provide for intersentential anaphora.  DRT solves


          that problem by creating a unified representation for the

          discourse, so that all markers in the discourse are available for

          anaphoric relations (with exceptions to be treated shortly). 

          Conflict resolution is not treated beyond the recency heuristic;


          this is not a weakness particular to DRT, for a full treatment of


          









                                                                          4


          pronominal anaphora must take into consideration grammar,

          pragmatics, and knowledge of the real world.  Definite noun

          phrases perform as do personal pronouns but, since they carry

          more content, are less likely to introduce conflict.  (Definite


          NPs used generically are not considered.)

               Proper nouns and indefinite NPs introduce new reference

          markers into U.  Although this procedure corresponds to the

          "first glance" view of natural language, it encounters problems


          in sentences that involve negation, disjunction, or conditions:


          (3)       Pedro does not own a donkey.

          (4)       Pedro owns a donkey or a cow.


          (5)       If Pedro owns a donkey he beats it.


          One certainly would not want to add a reference marker for

          'donkey' in (3); the semantics would require that it map to an


          entity in the model, and the sentence explicitly denies its

          existence.  The same holds for the donkey in (4), since it has

          perhaps a 50-50 chance of existing (although it might be useful

          to add a marker for the thing that Pedro owns).  Sentence (5) is


          the so-called "donkey sentence"; it not only introduces a donkey

          that may or may not exist, but goes on to make anaphoric

          reference to it.

               DRT handles sentences of the above types by adding to C one


          or more sub-DRSs.  Each sub-DRS has its own universe, which is

          not visible to the superordinate DRS, and its own condition set,

          and the truth value of the sub-DRS is determined by the logical

          connective that controls it.


















          









                                                                          5






          (3')      K:<U:{R1}

                       C:{Pedro(R1), ŠK',

                          K':<U:{R2}, C:{own(R1, R2), donkey(R2)}> }>

          (4')      K:<U:{R1}


                       C:{Pedro(R1), K' or K'',

                          K':<U:{R2}, C:{own(R1, R2), donkey(R2)}>

                          K'':<U:{R3}, C:{own(R1, R3), cow(R3)}> }>

          (5')      K:<U:{R1}


                       C:{Pedro(R1), K' -> K'',

                          K':<U:{R2}, C:{own(R1, R2), donkey(R2)}>

                          K'':<C:{beat(R1, R2)}> }>



          Thus, (3) is true if there is no entity in the model that

          satisfies its universe and conditions, (4) is true if there is a

          successful mapping from one of its sub-DRSs to the model, and (5)

          is true if any mapping that satisfies the antecedent DRS also


          satisfies the consequent DRS.

               Sentences (4) and (5) introduce an additional problem; each

          could be followed by a sentence such as (6):



          (6)  It is unhappy.


          The pair (5-6) is handled by including (6) in the consequent DRS

          for (5).  The other pair, (4-6), seems to be overlooked by


          theorists, but it can be handled, as was suggested above, by

          adding to the main DRS a marker for the thing that Pedro owns and

          including only the properties -- donkey or cow -- in the sub-

          DRSs.


               Universal propositions have the same DRS form as

          conditionals.  The scope of a universally quantified term

          relative to an existentially quantified term is indicated by the

          U in which the existentially quantified term is placed.  Thus,


          the usual interpretation of (7) is represented by (7a), while the

          interpretation that places 'donkey' outside the scope of 'farmer'

          is shown in (7b):



          (7)       Every farmer owns a donkey.


          









                                                                          6


          (7a)      K:<C:{K' -> K'',

                          K':<U:{R1}, C:{farmer(R1)}>

                          K'':<U:{R2}, C:{own(R1, R2), donkey(R2)}> }>

          (7b)      K:<U:{R2}, 


                    C:{donkey(R2), K' -> K'',

                          K':<U:{R1}, C:{farmer(R1)}>

                          K'':<C:{own(R1, R2)}> }>



               DRT, as described so far, does a very good job of handling a

          very small subset of English sentences.  Kamp and others have

          offered a number of extensions to the basic theory in order to

          expand that subset.



          2.   Extensions to the Basic Theory


               The basic theory is confined to a very limited subset of


          natural language.  In particular, it is limited to singular, non-

          generic NPs, to anaphoric reference (i.e. the referent is present

          in the discourse), and to sentences whose main verbs do not take

          propositions (i.e. DRSs) as arguments.  Researchers have offered


          extensions to the basic theory that reduce the second and third

          of those limitations.

               Kamp 1983 and Pinkal 1986 have offered refinements to the

          reference-resolving algorithm for definite NPs that extend the


          power and accuracy of that algorithm.  Kamp distinguishes four

          kinds of definite noun phrases (Pinkal: 369):


          (8a)      Personal and possessive pronouns


          ( b)      Complex demonstratives.  (Demonstrative + NP; NP may

                    include a restrictive relative clause.)

          ( c)      Definite descriptions. ('the' + NP; NP may include a

                    relative clause.)


          ( d)      Functional definite descriptions. ('the' + NP +

                    prepositional phrase, the latter limiting the set from

                    which NP selects.)



          Complex demonstratives differ from definite descriptions in that

          the latter presuppose a unique referent while the former

          presuppose a contrast between two or more possible referents. 

          Resolution of referential expressions requires the following



          









                                                                          7






          (Pinkal: 370):


          (9a)      The DRS K.

          ( b)      A salience ranking of the markers in UK. (Including


                    recency of reference.)

          ( c)      A selection set of UK whose members are available as

                    antecedents.

          ( d)      The universe of the real world needed for deictic


                    reference.


          Pinkal argues that definite descriptions are not limited to the

          selection set and that there is no motivation for the distinction


          between anaphora and exophora (where the referent is not present

          in the discourse; it is either physically present--deixis--or

          present in shared knowledge).

               Guenthner et al. 1986 extend the basic theory by adding two


          new types of discourse markers: event markers and time markers. 

          They include meaning rules in the DRS construction algorithm that

          assign each verb and each noun that refers to an action (e.g.

          'accident') to an event marker.  Each time reference (i.e. time


          of day or extent of duration) is assigned to a time marker. 

          Events are temporally ordered with respect to each other and to

          time references: an event may precede or overlap another event or

          time, it may be given a time argument expressing its duration,


          and it may be a subset of another event.  The addition of event

          markers makes it possible for predicates to take DRSs as

          arguments. Guenthner et al. do not include any examples of such a

          use of event markers, but Guenthner 1987 does.  In that article


          he also makes a notational distinction between events, which

          advance the time of the discourse, and situations or static

          verbs, which do not.

               Spencer-Smith 1987 does not use event markers, but adds a


          different type of discourse marker, a proposition marker.  This

          extension makes it possible to include embedded predicates, such

          as infinitival complements and beliefs:



          (10)      Mary wants to marry a rich man.


          









                                                                          8


                    K:<U:{R1, P1},

                       C:{Mary(R1), want(R1, P1),

                         P1:<U:{R2},

                             C:{rich(R2),


                                man(R2),

                                marry(R1, R2)}> }>


               The representation of beliefs, which is explored more fully


          in Kamp 1985, requires two further additions to DRT: internal and

          external anchors.  Anchors are used to associate discourse

          markers to entities in the world.  External anchors are ordered

          pairs, <Marker, Entity>, that associate the two as they actually


          are, while internal anchors are DRS-like structures that

          associate items as the speaker believes they are.  The use of

          anchors makes it possible to represent propositions that are in

          fact contradictory but are not so in the speaker's belief system


          because his internal anchors differ from the external anchors:


          (11)      John believes that Hesperus is pretty and Phosphorus is

                    not pretty.


                    External anchors: <R1, John>, <R2, Venus>, <R3, Venus>

                    Internal anchors: 

                         K:<U:{R2, R3},

                            C:{evening_star(R2),


                               morning_star(R3)}>

                    K:<U:{R1, P1},

                       C:{John(R1),

                          believes(R1, P1),


                          P1:<U:{R2, R3},

                              C:{pretty(R2),

                                 ŠK',

                                 K':<C:{pretty(R3)}> }> }>



               These extensions to DRT give it considerable power, but are

          far from giving it the power necessary to represent adequately

          the full range of meanings available in natural language.  In the


          next section we attempt to apply DRT to a selection of natural

          language in order to discover further extensions that will be

          necessary if DRT is to become an adequate theory for the

          representation of natural language.



          









                                                                          9






          3.   Application of DRT


               The passage to be analyzed here was treated extensively in

          Smith 1977 in order to determine the types of information that


          must be added to the text in order to obtain a complete

          representation of the situation reported by the text.  The text

          is a narrative passage that has been normed at sixth-grade

          readability (ETS 1969).  It is particularly interesting because


          it forces the reader to treat certain items as if they were in

          K0.

               In order to represent this passage, it is necessary to

          postulate ad hoc extensions to DRT.  Although these extensions


          work for this passage, they should be regarded as suggestions

          only and not as fully developed extensions; some will reveal

          their weaknesses as the representation is developed.

               The DRS K for the passage will be developed incrementally,


          the DRS for each portion being added to the existing DRS.  The

          clean-up of redundant discourse markers, however, is assumed to

          take place before the DRSs are combined.  Additional symbols will

          be explained as they are introduced.  As before, discourse


          sentences will be presented in the company of the DRSs that they

          add to K; since these sentences, unlike those in previous

          examples, have a cumulative effect, they will be denoted with the

          prefix N.


               In order to treat reference adequately, the items shown

          below must be included in DRS K0.  These items are, in effect,

          imposed on the reader as possible referents.  The marker Now

          indicates the time of reading.


           












          









                                                                         10


          (N0)      K:<U0:{R1, R2, Now, R10, R15},

                       C0:{}>


          (N1a)     The cave widened out as he went


                    U := U0 + {E1, E2}

                    C := C0 + {cave(R1),

                               E1:widen_out(R1),

                               E2:go(R2),


                               E2 o E1,

                               E2 << Now }


               The symbol o indicates that E2 overlaps E1; << indicates


          that E2 (and therefore E1) precedes the time of reading. 

          'widened out' is treated as a unit verb; the 'out' is actually

          redundant.  Since 'the cave' is definite, its referent must exist

          prior to (N1); for this reason R1 is included in U0, and the same


          is true of 'he' and R2.


          (N1b)     and the bottom seemed to drop away little by little

                    U := U + {R3, E3, P1}


                    C := C + {bottom(R3),

                              part-of(R3, R1),

                              E3:seem(P1),

                              P1:<U:{E4},


                                  C:{E4:drop_away(R3),

                                     little_by_little(E4)

                                     E4 =< E3}>

                              E2 o E3}



               R3, 'the bottom,' has no apparent antecedent and might have

          been included in U0.  It seems more likely, however, that it

          existed implicitly and that a meaning rule (such as 'Every


          physical object has a bottom') resolves the reference.  E3 is

          true if P1 seems to be true, even if P1 is actually false.  Since

          E4 is controlled by 'seem,' it is a subset ( =< ) of E3.  (I am

          not sure that this is what Guenthner et al. mean by subset, since


          they offer no examples, but it works here.)






          









                                                                         11






          (N1c)     and then, with no warning, it split in two directions,

                    U := U + {E5, Set1}

                    C := C + {ŠK1c

                              K1c:<U:{R4},


                                   C:{warning(R4)}>

                              E5:split_in(R1, Set1),

                              E3 << E5,

                              direction(Set1)}



               Since there is no warning, R4 is not visible to the top-

          level K.  'directions' introduces what is perhaps the major

          weakness in current versions of DRT, a means of representing


          plural nouns.  The ad hoc solution offered here is to use set

          markers, following a suggestion in Guenthner et al.  The

          proposition direction(Set1) is a notational shorthand for a

          complex sub-DRS representing "All members of Set1 are


          directions."



























          









                                                                         12



          (N1d)     one path leading straight ahead and one off to the

                    left.

                    U := U + {E6, R5, R6, E7, R7, R8}


                    C := C + {path(R5),

                              E6:lead(R5, R6),

                              R6 <- Set1,

                              straight_ahead(R6),


                              E6 =< E5,

                              path(R7),

                              E7:lead(R7, R8),

                              R8 <- Set1,


                              to_the_left(R8)

                              E7 =< E5}


               The cohesion of R5 and R7 with R1 is indicated by the fact


          that R6 and R8 are members of ( <- ) Set1.


          (N2)      "If I were an opening to this cave, where would I be?"

                    he asked himself.


                    U := U + {E8, P2, R9}

                    C := C + {E8:ask(R2, R2, P2)

                              P2:K2a -> K2b,

                              K2a:<U:{R9}


                                   C:{opening(R9),

                                      part-of(R9, R1),

                                      R2 = R9,

                                      R2 \= R9}>


                              K2b:<C:{location(R2, ?)}>

                              E7 << E8 }


               (N2) is, on the one hand, almost ridiculous; its only


          contribution to the understanding of the passage is the knowledge

          that 'he' is lost (Smith 1977), but that knowledge is no more

          explicit in the DRS than it is in the sentence itself.  On the

          other hand, it is a major headache for DRT.  (N2) is an embedded


          contra-factual conditional whose antecedent is impossible and

          whose consequent is a rhetorical question (indicated by the ? as

          an argument to location).  Its embeddedness, in this case, is

          wrong, in the sense that it is not a matter of 'his' belief, but



          









                                                                         13






          in another situation it might be.  The implication itself is

          worthless, but another implication might not be.  The conclusion

          that R9 is not R2 (indicated by \= ) is obvious but might be

          useful in another contrafactual.  The whole DRS must be added to


          K so that the reader can infer, by conversational implicature,

          that 'he' does not know the answer to the rhetorical question and

          that, since he does not know the answer, he is lost.



          (N3)      Luke wasn't frightened.

                    U := U + {Sit1}

                    C := C + {Luke(R2),

                              ŠSit1,


                              Sit1:<C:{frightened(R2)}>

                              Sit1 o E8}


               (N3) introduces a situation (more accurately, a non-


          situation) whose duration is vague but which at least overlaps

          E8.


          (N4a)     Oh, he knew there were such things in this world as


                    bottomless caves,

                    U := U + {Sit2, P3}

                    C := C + {Sit2:know(R2, P3),

                              Sit2 o Sit1,


                              P3:<U:{Sit3, Set2, R10}

                                  C4:{bottomless_caves(Set2),

                                      world(R10),

                                      Sit3:exist_in(Set2, R10),


                                      Sit3 o Sit2}> }


               P3 is like an external anchor, in that it is a fact about

          the world, but Kamp 1985 does not allow for propositions as


          external anchors.  It could be treated as an internal anchor, but

          it is explicit in the discourse.  Both Sit2 and Sit3 are true

          throughout the discourse, so they are irrelevant as temporal

          markers, but either might have changed during the discourse and


          the representation must allow for that possibility.


          









                                                                         14


          (N4b)     where people fell in and were never heard of again,

                    C4 := C4 + {K4a -> ŠK4b,

                                K4a:<U:{Set3, R11, R12, E9}

                                     C:{people(Set3),


                                        R11 <- Set2,

                                        R12 <- Set3,

                                        fall-in(R12, R11)},

                                K4b:<U:{R13, E10},


                                     C:{R13 <- Set3,

                                        R13 \= R12,

                                        E10:hear_of(R13, R12),

                                        E9 << E10}> }



               This is a continuation of the sub-DRS begun in (N4a); it is

          interpreted as a universal: 'No person who falls in such a cave

          is ever heard of again.'  Since this universal is embedded in a


          belief, it does not matter whether such persons exist or not; if

          it were not, it would be necessary to replace R11 with a set of

          at least two members.



          (N4c)     but if there had been any such thing around the cottage

                    he would have heard about it.

                    U := U + {Set2}

                    C := C + {K4c -> K4d,


                              cottage(R15),

                              K4c:<U:{R14, Sit4},

                                   C:{R14 <- Set2,

                                      Sit4:exist_around(R14, R15),


                                      Šexist_around(R14, R15)}>

                              K4d:<U:{E11},

                                   C:{E11:hear_about(R2, R14),

                                      E11 << E1}> }



               It is not clear whether (N4c) should be treated as a

          continuation of the belief initiated in (N4a), as a different

          belief, or as a top-level condition.  Viewed objectively, it is a


          belief (and an illogical one at that), yet it does not seem to be

          syntactically embedded in 'know,' or any other verb of belief. 

          If it is a different belief, or a top-level condition (as it is

          treated here), Set2 must be promoted from P3 to the top-level so



          









                                                                         15






          that it can be visible to other sub-DRSs.  R15 must be added to

          K0; it is a definite description whose referent cannot be deduced

          in the way 'bottom' can be deduced as 'part-of' a cave.  As with

          K2a, it would seem reasonable to elevate the negation of the


          antecedent of a contrafactual to the top-level, but in that case

          R14 would not be accessible (it would exist in a subordinate

          universe).  Since Sit4 is a general proposition, no temporal

          relation is assigned; the same is true of Sit5, Sit6, and Sit7


          below.


          (N5a)     This was just a plain, ordinary cave--deeper than most,

                    but that was all--


                    U := U + {Sit5, Sit6, Set4}

                    C := C + {Sit5:plain_cave(R1),

                              Sit6:ordinary_cave(R1),

                              just(Sit5),


                              just(Sit6),

                              cave(Set4),

                              ŠK5,

                              K5:<U:{Set5, R16},


                                  C:{Set5 =< Set4,

                                     cardinality(Set5) >

                                        cardinality(Set4)/2,

                                     R16 <- Set5,


                                     R1 <-\- Set5,

                                     deeper_than(R16, R1)}> }


               From a logical point of view, most of (N5a) is redundant;


          the only useful part is 'deeper than most,' and that belief lacks

          credibility.  Nevertheless, it poses several problems: handling

          the adjective-common noun combination, handling 'just,' and

          accounting for 'most' in a manner suitable for logic.  The


          adjectives 'plain' and 'ordinary' (unlike 'red,' e.g.) have

          little meaning until applied to a particular domain-- caves, in

          this case.  The adverb 'just' means something like 'not other

          than' in this case, but how is that meaning determined?  It does


          not seem to be a syntactic matter, but a DRS is composed from a


          









                                                                         16


          syntactic parse.  K5 is an attempt to handle 'most'; the symbols

          < and / have their usual mathematical meanings; <-\- indicates

          'not a member of.'



          (N5b)     and some place there had to be an opening to it.

                    U := U + {R17, R18, Sit7, P4}

                    C := C + {place(R17),

                              opening(R18),


                              part-of(R18, R1),

                              Sit7:necessary(P4),

                              P4:<C:{located(R18, R17)}> }



          The truth of (N5b) is doubtful, but given its truth, R17 and R18

          must exist at the top-level.  It is possible that R18 is

          identical to R9, now raised to top-level.  Sit7 suggests one way

          to handle modal auxiliaries.



          (N6)      There was, though, one big difference about this cave:

                    it was Luke's.

                    U := U + {Sit8, Sit9}


                    C := C + {Sit8:difference(R1, Set5, Sit9),

                              Sit9:own(R2, R1),

                              Sit8 o E1,

                              Sit9 o Sit8}



               The noun 'difference' entails two things that are different

          (this cave and other caves) and the thing that distinguishes them

          (Sit9).  However, only Sit9 is syntactically specified.


















          









                                                                         17







          (N7)      He had found it and it was his own secret place.

                    U := U + {E12, Sit10, R19}

                    C := C + {E12:find(R2, R1),


                              E12 << E1,

                              secret_place(R19),

                              R19 = R1,

                              Sit10:own(R2, R19)


                              E12 << Sit10}


               The representation of (N7) is straight-forward; R19 is R1,

          but to replace it by R1 in Sit10 is to make Sit10 a copy of Sit9.


               This application of DRT has pointed out several needed

          extensions to the theory.  One of the most obvious is the means

          of representing plural NPs, including those with quantifiers that

          are less specific than 'all' but more specific than 'some' (e.g.


          'most').  Another needed extension is a means of handling terms

          that modify conditions: verbs that take verbals as complements

          (modals and verbs such as 'seem') and adjectives whose meanings

          depend on the particular nouns that they modify.  A third


          extension is a formalism for specifying arguments that

          are not syntactically indicated (such as those for 'difference'). 

          Whether or not the second and third extensions are feasible

          without appealing to semantic analysis prior to constructing the


          DRS remains to be seen; perhaps the needed machinery is available

          in the lexicon.


          Conclusion



               DRT has been successful in representing a small subset of

          natural language, and is being extended to increase the size of

          that subset.  As we have seen in Part 3 of this report, other


          extensions will be necessary before it can handle the full range

          of natural language expressions.   If those extensions can be

          accomplished without appeal to semantics, DRT will prove to be

          quite powerful.  However, DRT is intended to provide a bridge


          between syntactic parses and model theoretic semantics; if


          









                                                                         18


          semantic analysis is necessary before a DRS can be constructed,

          the purpose of DRT has been lost, or at least seriously modified.




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