EX'I~NDING Tfiq~ PlKPRESSIVE CAPACITY OF Tf~E SFIMANTIC (DMPONENT ~F ~\[~, 
OPFJ~A SYST~ 
CELESTIN SEDOGBO 
CENTRE DE I~ECH~RCHE BULL 
68, ROUTE DE VE, RSAILLES 
78430 \[~3UVECIENNES 
FRANCI~ 
A~kqTRAOI': 
OPERA is a natura\], language question answering syst~n 
allowing the interrogation of a (hta base consisting 
of an extensive listing of operas. The linguistic 
front-~d of OPERA is a c~nprehensive grm~r of 
Vrench, and \]1:s semantic c~nponent translates the syn- 
• tactic analysis into logical formu\]as (first order \]o-. 
gic forrnulas) .. 
However there are quite a flew constructions which 
can h~ analysed syntactically in the grmn\[mr }ant for 
which we are l\[~able to specify trans\]ations, t'~orcmlost 
muong these are m)aphoric and elliptic contructions. 
Thus this \[kqper descri\[~s the extension of! OPERA to 
anaphoric and elliptic c~mstructions on the basis of 
the Discourse Representation Th\[~)ry (DRT). 
l- \]INTRODUCTION 
OPEPJ~ is a natural language question answering system 
al\].owing the interrogation of a data \[mse consisting 
of an extensive listing of operas and their ass(x~\]ated 
caracteristics (c(~ll\[~sets~ time of ccm\]pos/.tJon v cecor-. 
dings, etc .... ). The \].inguishic front.-.~md o\[ OPF, I~A i.s 
a c<~nprehensJve grat~nar of Fwench based essentially 
on the string gra~ar for~m\].ism (cf for exc~n\[)le 
\[SAGE\[{~S1\] o~: \[SALKOFF~ 73\]); the details of the syn-~ 
tax used in OPE|{A are descri\[x:d in \[SEIIYOGBO, 85 \]. \[in 
\['his paper we shall descri\[x~" Lhe scmmrrh:i.c c~u\[mnent ol 
OPERA as it stands now. In addition we shall d:\[scuss a 
nun~x~,r <)if prob\].c:~natic constructions which are not 
handled at present in the systc{~ and we outline a 17os-. 
sible solution for thon in the setting of this system° 
It has bc~coale more ~/ld nDre C\]l ~k~\] \[ that the n~ost natu- 
ral and transparent fon~ulation of the s~imntics of 
natural languages can be constructed in terms of a lo.- 
gical so~Itics i.e a sel~%ntics b~sed on the usual mo- 
de\] theoretic interpretation of predicate logic. 
This was in fact already realized as early as 1973 by 
A. COI~4~:RAUER and his group ( CF \[COLMF/~AUER & ai,73\], 
\[PASERO, 73\], \[COI~MERAUER, 79\]). In these papers the 
sexnantics for natural languages is specified in terms 
of the truth conditions of predicate logic formulas 
into which these utterances are translated in a sys- 
t~natic way. ( It should be n~ntioned that in many 
respects the way these translations are obtai ned as 
well as the translations the~mselves resemble the even 
earlier proposals of MONTAGUE and his students (cf 
\[MONTAGUE, 74\]). The work done in COLMERAUER's group 
has however the additional merit that these proposals 
are presented in operative syst~ns for nmn-machino 
dialogues. Indeed in these early papers we find sket- 
ched two alternatives for the systs~mtic manipulation 
of the translations provided for natural language 
discourse: 
This research was supported in part by ESPRIT, in the 
ACORD project(P393). 
a) given a data base in relational (e.g Prolog like ) 
form the answers to queries (which are themselves 
translated into logical forms) ~n he given in terms 
of the satisfaction conditions of these formulas. I.e 
a query like "does every student own a hook? " will 
receive the answer "yes" if its translation " for a\].l 
x (student(x)-> there exists y ( book(y)& own(x,y))) 
" is true in the data ~ise. Here the answers to que- 
ries are obtained by simply evaluating the translation 
in the data base (or mode\].)° 
b) another approach however consists in regarding the 
data base as a set of fomaulas and the process of ans- 
wering a query as a deduction° A query wil\] be said 
to be true with respect to the ~ita \]mse if and only 
if it can I~ <\]educed frc~n \]to 
(Naturally as in the case of the first approach, this 
meth(x\] app\].ies (~qual\]y well to closed sentences ( i°e 
yes-no questions) as well as to open sentences ( i.e 
W\[I-questions)° \],'or an application of this im~thod as 
wo\].l as for disc:ussion of its advantages (cf \[PASERO~ 
73\])° 
\].'o~. reasons which w~ sha\]l not spell out here we have 
chosen the ~ode\] the(met\]ca\] approach (\].e the first 
approach ) in OPERA° Needless to say it: is not Loo 
difficult and in fact, sclnetimes necessary~ as we shall 
see below, to c<mlpl6\]nent the pure se~nantic evaluation 
with deductivo capacities.. 
2- Sh:MANTICS VIA TRANS~TION IN'IY) LOGIC 
Even if it is clear in nmst respects how to obtain 
predicate logic: translaticms for a large w~riety of 
natura\] language sentences, cf \[WAR\[~FM & PEREIRA, 82\] 
we repeat here :\[or the sake of clarity the essentials 
of the translation process in OPERA. 
Consider for c~ample the following sentences : 
(a) Berg a c<~npos6 un c~era en trois acres 
(Being composed an opera in three acts) 
(b) aucun c~upositeur gl.l~mnd n'a compos4 un opera 
en 1912 
(no german cc~poser composed an opera in 1912) 
(c) Chaque opera de Berg a dt@ ~\]registrd par Karajan 
every opera by Berg was recordcw\] bY I~rajan. 
The syntax of ~?ERA (cf \[SEDOGBO, 85\]) yields syntac- 
tic analysis as in (d). 
These trees are then translated into the following 
predicate logic formulas : 
(a) exist(x,opera-in-three-acts(x) & compose(Berg,x)) 
(b) aucun(x,composer(x) & german(x),exist(y,opera(y) 
& canpose-by(y,Berg) & in(y,1912))) 
(C) chaque(x,opera(x) & of(x,Berg), 
recorded-by(x,Karajan)) 
23 
,---,,---,,---,,,,---,, 
--V' ---AUX-=-a I 
---PAST---cf~npes4 
---OBJ---NP .... LN .... art---un I 
-CN---NO\[~---opera (d) I 
--PP---~rep---~l 
---NP---IN----QC~2---trois I 
--NOUN---actes 
Fiqure 1: an example of syntactic tree in OPERA 
The evaluation component of OPERA then evaluates these 
formulas in the data base. Quite a few syntactically 
complex queries can thus be formulated in this system. 
3- FROM TRANSLATIONS TO DRSs 
OPERA was conceived not so much as an effort towards 
an implementation of viable integrated natural langua- 
ge question answering system, as a testing ground for 
a rather extensive fragment of French. In fact many 
constructions ( in particular cc~plement 
constructions ) which are accemodated in the syntax 
fragn~nt of OPERA are such that they cannot effective- 
ly be applied in the setting of OPERA's data base. 
H(~ever there are quite a few constructions which can 
be analysed syntactically in the gra~ir but for which 
we are unable to specify translations, even when their 
intended translations are essentially first-order lo- 
gic formulas. Foremost among these are anaphoric and 
elliptical contructions. It is clear' that any c~npre- 
hensive treatment of such phenomena cannot restrict 
itself to the analysis of isolated sentences i.e to 
isolated queries. For what could a sentence like: 
when did he compose it ? 
mean ? or what query could be expressed by the phrase 
"and Britten?"? 
There is not much that these sentences could convey 
in isolation. But consider the following dialogue : 
Question: did Beethoven cazpose Fidelio ? 
Answer : yes. 
Question: when did he ccnkoose it? 
A/iswer: in 1812 
Or consider another very natural interaction : 
Question: how many operas did Beethoven compose? 
answer: 1 
Question: and Mozart? 
Answer : 15 
It is clear that an adequate account of anaphoric and 
elliptic constructions must at least take into account 
the current situation of the dialogue. How to define 
the notion " current situation" is by no means a tri- 
vial task. For one thing we cannot simply asssume that 
the current situation is identical to the sequence of 
the question-answer pairs that make up the dialogue. 
Much more is involved. 
3.1 TOWARDS A TREATMENT OF ANAPHORA 
24 
Even though pronouns raise no probl~u at all as for ~ 
as far as t~ syntax is concerned, no one has been 
able to provide a unified and systematic account of 
anaphoric linking. As recent work has shown one must 
distinguish on the one hand various types of pronc~i- 
hal reference and on the other hand show how the reso- 
lution of anaphora must appeal to common Lmderlying 
mechanisms. 
It is one of the achivements in \[KAMP, 81\] to show the 
way towards that unified treatment . 
In fact we shall base our proposals for beth the 
treatment of m\]aphora and of ellipsis on the concepts 
and technics introduced by KAMP in his theory of di- 
scourse representation. 
Since the current translation cx~mpenent of OPERA does 
not take into account pronouns at all we must first 
provide an extension which is also able to deal with 
pronominal referemce. Syntactically pronouns can occur 
in all NP positions characterized bY the grammar° 
We shall 1~mrk each occ\[mence of a pronoun by indica- 
ting its gender/number features; As a first step in 
the translation procedure we shall therefore simply 
take these translations to he identical with themsel- 
ves. Thus the translation of sentences like : 
Berg l'a composd 
(Berg c~mpesed it) 
will be : 
co, loose(Berg, \[it,null,sing\]) 
similarly a sentence like : 
chaque cc~npositeur qui a compos4 un opera l'a enre- 
gistr4 
(every ccTmposer who wrote an opera recorded it). 
will be translated into : 
chaque(x,exist(y,opera(y)&composer(x)&c~npose(x,y)), 
record(x,\[it,null,sing\]))) 
We shall call translations containing occurrences of 
pronouns "unresolved translations". Notice that 
contrary to the practice in \[MONTAGUE, 74\] we do not 
index pronouns either in the syntax or the initial 
tranlation phase. The resolution of anaphora will ta- 
ke as input "unresolve<\] translations" and yield as 
output a rather different type of s~nantic representa- 
tion namely a discourse representation structure 
(DRS). 
What is DRS? In general we shall say that a DRS is 
pair consisting of a (possibly empty) domain of di- 
scourse referents U and a set of conditions CON. We 
shall for the present discussion take into considera- 
tion only three types of conditions : 
a) atc~nic conditions, which consists of n-ary predi- 
cate P and terms. 
a term is either a discourse referent or a proper 
name; among the predicates 
we single out the eqdality predicate "=". 
b) conditional conditions which have the form 
=>(kl,k2) where kl and K2 are 
again DRSs. 
c) negative conditinal conditions which has the form 
#>(Kl,k2). 
In a more comprehensive treahnent Jt is clear that we 
shall need further types of conditions. 
Thus for the final translations of unresolved transla- 
tion we want to arrive at DRSs like the following : 
(a) 
Berg Loulou 
c~npose(Berg,Loulou) 
(b) 
--i--f ......... 
composer (x) 
opera (y) 
com~x\]se (x, y ) 
__> recerd(x,y) 
There are precise truth definitions for DRSs which we 
shall not s\[~ll out here however (cf KAMP for 
details). In any case the first DRS is logically equi- 
valent to the formula : compose(berg, loulou) and the 
second DRS is logically equivalent to the predicat lo- 
gic formula ~: 
chaque(x,(cha~ue(y,(cc~poser(x)&opera(y) & 
compose(x,y)) -> record(x,y))) 
What is interesting in the second example is of course 
a fact that the pronoun "it" has as its syntactic an- 
tecedent the noun phrase "an opera ". The se~qntic 
force of this noun phrase cannot however be rendered 
in terms of an existential quantifier at \].east not if 
we want to establish an ~maphoric \]ink between the 
existential quantifier and the variable representing 
the pronoun in the consequent of the conditional. 
MO~I~AGUE for instance runs into this proble{n in Vi~ 
where the pronoun is either left unbound or either is 
bound by the existential quantifier when the latter 
occur outsi~ the conditional all together. This wide 
scope reading of the indefinite NP obviously gives 
them the wr(mg interpretation. As does of course the 
translation leaving the pronoun unbound. In the DRS on 
the other hand we get a universal reading for an opera 
as the result of the interpretation of the conditio- 
nal. 
Even though inside the antecedent of the conditional 
we treat the occurrence of the indefinite noun phrase 
"an opera " as we would in an ordinary indicative sen- 
tence, where of course ~le interpretation of the in- 
definite article corresponds more naturally to an 
existential quantifier. This is one of the features of 
IQNMP theory that we shall tmke advantages of in the 
present proposal. 
As we said above we shall transform unresolved 
translations stepwise into DRSs. Needless to say we 
could of coucse set up the translation procedure in 
such a way that we obtain DRSs directly from the out- 
put of the syntactic component. But this would entail 
major revision of the entire translation algorithn in 
any event the way we propose to derive DRSs can he 
regarded as DRS construction algorithm in its own 
right. On this view "our unre~uced translations" play 
the role of an intermediate structure between syntax 
and DRSso 
Assume that t is an unresolved ~:anslation of a sen- 
tence . We first generate K(t) the discourse represen- 
tation structure corres~:mding to t, in the following 
way : 
if t is a univemsal tree i.e a formula of the fon~ 
chaque---x 
--fl 
---f2 
we create a DRS K(t) with an empty donmin and the 
condition => (KI,K2)o 
K1 will have x in his d~nain and the result of trans- 
forming fl in K1 as its condition 
K2 will have an empty d~min and the result of tran- 
:Forming :f2 as its c~onditions. 
Let t be a tree with the determiner "aucun" as its 
top node, i.e a formula of the form : 
aucun---x 
---fl 
.... f2 
we procede as in the case above except that the condi- 
tion we emter into K(t) is now a negative universal 
condition, i.e a condition of the form =>(kl,K2). 
Suppose t is dallinated by exist i.e t has the form : 
exist .... x 
.... fl 
we create a DRS K(t) whose domain contains x and we 
add the result of traulsforming fl as the conditions to 
K(t). 
Suppose t is a tree dc~inated by "et" 
of the form : 
et---fl 
---f2 
i.e a formula 
we create a DRS K(t) with an empty domain whose 
conditions are the result of transforming fl and f2 
with respect to K(t). 
Finaly suppose the unresolved translation is non 
quantifie~\] then we enter it as is into K and we add 
all occurrences of proper names into the domain of K. 
This provides the induction basis for the tranforma- 
tion. Let K' k~ a DRS with dc~min U' and conditions 
CON' and let t he a tree i.e a formula. The result of 
transforming t in K' is the application of the above 
three rules to t. When we no longer have any tree to 
process all conditions in the principle K i.e the DRS 
representing the sentence to be transformed, as well 
25 
as all conditions occurring in the sub-DRSs of K will 
now be conditions in the language of DRSs or atomic 
conditions containing occurrences of pronouns. 
How are these be eliminated ? 
Let us first consider an example the sentence 
"every composer dedicated an opera to a conductor that 
he has admired" 
has as its unresolved translation: 
chaqui---x 
fl: ---ccmposer(x) I 
f2: ---exist---y f 
---exist---z I 
---conducter(z) & 
opera(y) & dedicate(x,y,z) & 
admire(he,z) 
We indicate the contruction of the DRS stepwise 
K1 
(i) 
x 
composer (x) 
K2 
(2) 
K1 x 
composer (x ---> 
K2 y z 
opera(y) 
conductor(z) 
dedicate(x,y,z) 
admire(x,z) 
(3) 
3.2 INCORPORATING DRS CONSTRAINTS INTO TRANSLATIONS 
• o" 
We will give below the exact method for translating 
formulas of OPERA into DRSs. 
One of the most important features of DR-theory is the 
precise constraints on the antecedents of pronouns. 
Let K' be a DRS embedded in K (which is a DRS too); 
the antecedent of a pronoun occurring in K' is the 
list consisting of the union of U(K) and U(K') if and 
only if K is accessible to K'. 
For a precise definition of the notion of accessibili- 
ty cf \[GUENTHNER & I~HMAN,85\]. 
Let us now illustrate the notion of accessibility by 
giving the table of accessibility of the DRS above: ~ 
DRS accessible I 
K 
K1 (I) 
By the transitive closhre of the accessibility rela- 
tion we obtain for example all the possible antece- 
dents of a pronoun occurring in K2 (e.g a pronoun 
occurring in K1 cannot have as antecedent a referent 
of K2). 
For the clarity of what will follow, we will call 
"unresolved predicate" (abreviated UP) a predicate 
whose arguments included at least one pronoun. Then to 
resolve an UP, one must replace the pronoun arguments 
with appropriate referents accessible. 
The idea is to transport during the translation of 
formulas, a list of antecedents accessible according 
to DRSs constraints° 
How is this list to be constructed? 
As shown in (3.1) a universal tree is translated into 
a DRS K= =>(KI,K2); the antecedent list of K which we 
note L(K) is 6{npty. 
The antecedent list of K\] is L(K1) and L(K2) = U2 + 
L(K1) (we denote the union of sets by the symbol +). 
The existential tree is translated into K = \[U,CON\] 
and the antecedent list L(K)=U. 
Let f be a formula with "aucun" at its top node; f is 
translated into the DRS K ~>(KI,K2). The list of ante- 
cedents L(K) of K is empty and L(KI) is U1 and L(K2) 
=U2 + L(KI). 
Let us call a DRS containing "unresolved predicates" 
an "unresolved DRS". Thus each unresloved DRS is a 
pair of the form K = \[K,L(K)\] where L(K) is the ante- 
cedent list of the DRS K. 
To resolve an unresolved DRS, each unresolved predica- 
te (UP) must be resolved according to the following 
mule : 
for an unresolved DRS K = \[K,L(K)\] with K=\[U,CON\] a 
UP P of CON is transformed into the logical predicate 
P"which is obtained by unifying pronoun arguments of 
P ih L(K). 
• The example "every composer dedicated an opera to a 
conducter that he has admired" treated in session 3.1 
will illustrate how unresolved DRSs are resolved. 
Given the unresolved DRS K = \[K,nil\] 
with 
K= A>(kl,k2) , KI= \[Kl,x\], K1 = (x,ccrnposer(x)) 
and 
28 
K2 = \[K2,(x,y,z)\] with K2= ((y,z,he),CON2), CON2 = 
opera (y) & conducter ( z ) & 
dedicate(x,y,z) & admire(x,he) 
K will he resolved by application of the rule descri- 
bed above, i.e: The pronoun he is \[mifiable in the 
list (x,y) of antecedents, to z. Therefore the unre- 
solved predicate will be translated into a~idre(x,z). 
4- EX'I~NDING THE DIALOGUE CAPACITY OF OPERA 
A proble~ arises in interrogating a data base in natu- 
ra\]. language which is the problem of dialogue situa- 
tion. For example : 
(a) qui a c~t~posd Loulou (who composed \[mulou ?) 
(b) oh est-il n4? (where is he born?) 
This dialogue can he translated into a DRS containing 
the semantic representation of the two sentences. But 
if there is no interaction with the data base, the 
anaphoric link of the pronoun in (b) will be an un- 
bound variable and not for example the individual 
"Berg" (as Jt is Berg who c~nposedLoulou). 
What we prefer for a question-answering system is to 
take into account the situation of a question and its 
answer. 
A possible solution could be. to consider that after 
the eval\[mtion of a query, its c~rresponding DRS is 
therefore instantiated (i.e. its unbound variables are 
now bound); in such a situation we loose the reading 
of the DRS as a logical formula and moreover we cannot 
represent the instantiation of a splitted DRS (e.g. a 
universal formula). 
4°1 q~E NCZI'ION OF A "CURRENT SITUATION DRS" 
The solution we proposed is to separate the DRS of a 
formula from the DRS for the situation. 
Let t be a formula and K(t) :\[U,CON\] its translation 
into a DRS. 
The DRS of situation is the ORS K§(n) = \[U§,CON§\] 
with 
U§ containing the instantiation of referents of 
U, 
CON§ (~pty, and n denoting the current state 
of the dialogue (i.e. the occurrence of the 
question during the dialogue). 
The rule for UP resolution presented in the last ses- 
sion, must now be modified in the following way: 
let n be the current state of the dialogue and K be 
an unresol~<\] DRS; the antecedent of a pronoun occur- 
ring in K is containel in the list consisting of the 
concatenati(xl of L(K) and L(K§(n-I)). 
The antecedent list of K§(i) is defined in the same 
way as that a noraml O~5, i.e. L(K§(i))=U§(i). 
To illustrate what is abever we will treat the dialo- 
gue above: 
(a) is translated into the logical formula 
tl= Wh(x,c~\]npose(x,Loulou)); its translation into a 
DRS will produce K1. 
K1 
?x Imulou 
compose ( x, Loulou ) 
Since there is no prono<~n in KI, we can then evaluate 
the formula tl. After evaluation, we can build the 
DRS of situation K§: 
KI§(1) 
Berg Loulou 
(b) will be tranlated into the logical formula 
t2 = Wh(y,bern( \[he,mas,sing\] ,y) ) 
The unresolved DRS of t2 is K2 =\[K2..L(K2)\] 
with 
K2 
\[he,mas,sing\] y 
bern(he,y) 
and L(K) = y + L(K§(2-1)) = \[y,Berg,Loulou\]. 
To resolve K2, an antecedent must be substituted to 
"he"; this antecedent will be Berg (because argtunents 
in OPFJtA are typed). 
The DP, S of situation becomes : 
K§(2) 
Berg Vienne 
4.2 TREAI\[MENT OF ELJ~IPSIS 
One of the most con~aon \]phenomena of dialogue is ellip- 
sis such as in : 
(a) Berg a c~pos6 loulou 
Berg ca~Iposed Loulou 
(b) Britten aussi 
Britten too 
or 
(c) et m%e symphonie 
and a symphony 
The interpretation of (b) is "Britten a cQuposd 
Loulou" (i.e. VP-ellipsis); and (c) is to be interpre- 
ted as "Berg a compos6 tme symphonie". 
The ad hoc treatment proposed in \[SEDOGBO, 85\] fails 
in 1lmny cases; as we mentioned, since the logical for- 
mulas in OPERA are equivalent to first-order logic 
formulas. 
An interesting extension to VP-ellipsis in DR-theory 
is described by KLEIN (cf \[KLEIN 84\])° 
KLEIN introduced the notion of predicate-DRS and pro- 
27 
posed an indexation of NP predicate-DRS and VP 
predicate-DRS in a DRS. 
We will not propose here how this extended DRS can be 
implemented; but will exploit the parallelism between 
our logical formulas and DRSs. 
Each sentence will be translated into two partial- 
logical formulas (noted PLF) of the form <x,f>, where 
x is a variable or an individual and f a logical for- 
mula. 
We assume that the composition of PLF(NP) and PLF(VP) 
gives the translation of the sentence. 
The first PLF is implicitly indexed by the NP and the 
second PLF is indexed by the VP. 
The DRS of current situation must be modified in the 
following way : K§(n)= \[U§,CON§\] with 
U§ defined as above, and CON§ containig PLF(VP). 
Given a VP-ellipsis s, it will be translated into the 
the PLF: <i,fl>. The DRS of current situation 
contains in its CON§ a PLF :<j,f2>. 
The VP-ellipsis is treated in the following way: 
i) ~lification of i and j 
2) the c~upositi~ of <i,fl> and <j,f2> produces the 
translation t 
3) t is translated into a DRS K and K§ is built as 
described in session (4.1) 
We will illustrate the VP-ellipsis treatment by pro- 
cessing the dialogue above: 
(a) is translated into the formula 
compose(Berg,Loulou) and PLF(NP) = <Berg,Berg>, 
PLF(VP)= <i,cempose(i,Loulou)>. 
The translation t of (a) is then obtained by ccraposi- 
tion of PLF(NP) and PLF(VP); thus 
t: compose(Berg,loulou). 
The evaluation of t will augment the dialogue situa- 
tion of a new DRS of situation containing PLF(VP). 
The translation of (b) will produce a PLF(NP)= 
<Britten,Britten> and a PLF(VP)= <i,p>. 
<i,p> will then be unified to the PLF(VP) contained in 
the CON§ of K§ , i.e <i,p> is unified with 
<i,ccmpose(i,Loulou)>; the composition of the two PLF 
<Britten,Britten>, <Britten,canpose(Britten,Loulou)> 
will produce the logical formula 
"ccmpose(Britten,Loulou)". 
5- CONCLUSION 
The extensions to the OPERA system proposed here give 
a powerful dialogue capacities to OPERA. 
On the basis of the DR-theory, we propose an extension 
for the treatment of anaphora. We do not treat here 
the definite article as a definite refence since in 
our system the definite article must be interpreted as 
an indefinite article. However notice that even if in 
the framework of DR-theory the definite article is ex- 
plained as a definite anaphora, its anaphoric link re- 
quires often the use of deduction. We prefer therefore 
not to treat the definite anaphora. 
28 
The extension toellipsis described in this paper is 
limited to VP-ellipsis; in fact the other \]dnds of el- 
liptic sentences can be seen as conjoined to the pre- 
ceding sentences. Even if the treatment proposed is 
not the one described by KLEIN, the notion of partial 
logic formula is equivalent to that of partial-DRS. 
In order to handle the dialogue we introduce the no- 
tion of a current situation DRS. 
But, is the level of logic translations (i.e our logi- 
cal formulas) necessary, since we translate these for- 
mu\]ms into DRSs? 
The reason to maintain this intermediate representa- 
tion is that our use of DRSs is only justified by the 
accounting of the dialogue, so that we do not need the 
complex features of a DRS system. Then for reason of 
efficiency we think that it is better to evaluate 
translations on the data base (this enables the t~e of 
an optimization algorithm before executing the 
queries). 
ACKNOWLEDGMENTS 
the author would like here to thank F. GUENTHNER for 
his help during the specification of these extensions 
and for his conments on this paper. 

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