Temporal Structure of Discourse* 
Irene Pimenta Rodrigues 
Josd Gabriel P. Lopes 
Centro de Intelig6ncia Artificial, UNINOVA 
Quints da Torte, 2825 Monte da Caparica Portugal 
email:(ipr,gpl)@fct.unl.pt 
Abstract 
In this paper discourse segments are defined and a 
method for discourse segmentation primarily based 
on abduction of temporal relations between seg- 
ments is proposed. This method is precise and com- 
putationally feasible and is supported by previous 
work in the area of temporal anaphora resolution. 
1 Introddction 
B. Webber in \[22\] explains how discourse seg- 
mentation contributes to the interpretation of 
tense. In this paper we discuss how "tense in- 
terpretation" contributes to discourse segmen- 
tation. Following Webber's operational defini- 
tion of segments \[21\] we present the data struco 
tures necessary for representing discourse seg- 
ments and an algorithm to perform discourse 
segmentation. 
In order to build a discourse segment struc- 
ture some clear criteria for grouping chunks of 
text into segments and for grouping these seg- 
ments into other segments must be available. 
Our criteriou is based on the maintenance of 
temporal coherence between segments. It relies 
on the abduction of temporal relations between 
segments that necessarily have temporal prop- 
erties. 
Abduction of temporal relations between seg- 
ments is a feasible and precise method for dis- 
course segmentation. This is the leading cri- 
terion for segmentation and does not prevent 
us from using other criteria such as clue words, 
spatial relations and recognition of state elabo- 
ration. Current discourse structure theories use 
criteria such as rhetorical relations \[9, 15, 13\], in- 
tention recognition \[7\], narrative discontinuities 
\[18\], etc. All of them use a temporal criterion 
for segmentation embedded ill less manageable 
*This work has been supported by JNICT, INIC and 
Gabinet¢ de Filosofia do Conhecimento. 
criteria such as "the increasing desire of R to 
perform action N "l. 
Our discourse segmentation is accomplished 
in order to enable us to address the following 
discourse phenomena: 
• Temporal anaphora - the interpretation of 
tense and aspect depends on the discourse struc- 
ture \[22\]. The maintenance of a "Temporal Fo- 
cus" is suggested by some authors \[11, 22, 18, 5\] 
for tense interpretation. Based on their work 
our main concern is to provide the "temporal 
focus" for tense interpretation. In our segment 
structure the temporal focus is the set of visible 
segments. A pop in the temporal focus structure 
will imply the closing of one or more segments. 
• This and that anaphora - These pronouns 
may refer to discourse segments \[21\]. With our 
segmentation we provide discourse segments for 
reference. 
• Pronominal and definite anaphora - the in- 
terpretation of pronouns and definite nominals 
depends on the attentional structure (Grosz and 
Sidner). The attentional structure depends on 
the discourse segmentation. With our approach 
to segmentation the attentional structure can be 
computed from our discourse structure -- our 
visible segments provide visible entities and se- 
quents for these anaphors. 
• event reference - our approach provides a 
representation for eventualities, the discourse 
referents and tile attentional structure necessary 
for solving this kind of anaphora, 
• temporal coherence - is achieved by in- 
ferring one of the possible temporal relations 
between two eventualities \[14\]. Our segmen- 
tation process mainly controlled by abduction 
of temporal relations between eventualities en- 
ables us to check if a text is temporally coherent. 
Moreover as we propagate temporal constraints 
through discourse structure the number of even- 
I I)efinition of the rhetorical relation motivation \[15\]. 
ACTES DE COLING-92, NANTES, 23-28 hO'dT 1992 3 3 1 PROC. OF COLING-92, NANTES, AUO. 23-28, 1992 
tualities that must be temporally related with a 
new eventuality increases. 
* temporal reliability - is achieved by the ex- 
istence of a model for the temporal relations in- 
ferred. During discourse processing we build a 
temporal structure where all the temporal con- 
stralnts can be checked. This structure is up- 
dated when a new temporal referent or a newly 
abducted temporal relation is added. Thus tem- 
poral reliability is granted. 
• discourse coherence - is difficult to check by 
using only our discourse structure. It requires 
more processing, namely the ability to find in- 
tentions for the segments. 
Tense interpretation contributes to segmenta- 
tion by defining the temporal relation between 
the segment used as reference and the segment 
that represents the tensed sentence to be in- 
terpreted. Thus tense interpretation allows the 
choice of a segment by indicating where to at- 
tach the new sentence segment and greatly re- 
stricts the possible referents for anaphora res- 
olution. A failure in satisfying structural con- 
straints results in the choice of another segment 
referent. 
The temporal anchoring of eventualities as- 
sumes that there is some temporal represen- 
tation for the eventualities. In this paper we 
use a representation for eventualities close to 
the event calculus\[12\], and a graph structure 
for time representation. Updating the discourse 
structure will be equivalent to updating a tem- 
poral data base. Discourse referents are exis- 
tentially quantified variables that can be further 
constrained by the analysis of a new discourse 
sentence. 
In the following sections the temporal rela- 
tions used in this paper are defined, followed by 
an explanation of our notion of segments, their 
properties and the algorithm for discourse seg- 
mentation. A detailed example will be worked 
out. Finally a conclusion and a comparison of 
our work with related work in this area is pre- 
sented. 
2 Temporal relations 
The semantic representation of an eventuality 
includes a time interval where the eventuality 
must be true so that the sentence and the dis- 
course can be true. The time interval will be 
represented by two time points. Temporal re- 
lations between two time intervals can be ex- 
pressed by relations between the extremes of the 
time intervals. So instead of using the 13 rein- 
tions proposed by Allen \[1\] we have chosen to 
use just 5 relations. 
• t,, < t, 2 = t,~s < t,~, - this relation is like 
Allen's relation before or meets. 
• t,, > t,~ --=- ts2s < i,,~ - this relation is like 
Allen's relation after or met-by. 
• t,, C t.~ = t.~, < t,,,,t,,s < t,~s - this 
relation is like Allen's relation IN. 
• to~ D t,3 =-- t°~+ < t,~,,t,~ < ts~s - this 
relation is like Allen's relation ON. 
• t°~ c~ t,= = 3t : t C t,,, t C t,2 - this relation 
is like Allen's relation !. 
These 5 relations are enough for translating 
natural language sentences as it is difficult to 
express the 13 relations of Allen and their 213 
combinations in natural language. F. van Eynde 
\[6\] presents the set of relations necessary for 
the temporal systems he studied (for EEC lan- 
guages). Our relation set, however, is smaller. 
As sentence aspect coerces the verb aspectual 
class to change \[16\], v. Eynde's overlap relations 
may be rewritten using the relations < and >. 
The following examples will demonstrate our 
use of the temporal relations. For the sake of 
simplicity only discourse referents introduced by 
eventualities and time intervals are represented. 
Eventualities introduced by nominals (as in ex- 
ample I "his key") are discarded. 
I) John picked off his key(l). He opened the 
door(2). 
The eventualities of sentences (1)and (2) are: 
event(st, pick(john, key1}}, time(eht,~); 
event(e2, open(john, door1)), time(e2,t°2); 
the temporal relation is t,, < t+ 2. 
lI) John bought an umbrella(l). He had lost his 
umbrella(2). 
The eventualities of sentences (1) and (2) are: 
event(el, buy(john, utah1)), time(eht°,); 
event(e2, looseOohu, umb~)) , time(e2,t°~); 
the temporal relation is ts~ > t,,. 
III) John bought an umbrella (1). It was rain- 
ing(2). 
The eventualities of sentence (1) and (2) are: 
event(et, buyOohn,umbl)) , time(ea,t,~); 
event(e2, rain), time(e2,t,,); 
the temporal relation is t,, C t,2. 
IV) John had a nice meal(l). He ate salmon(Z). 
The eventualities of sentence (1) and (2) are: 
event(el, have(john, meal)), time(el,t°t); 
eventCe2, eat(john,salmon)), time(e2,t,,); 
the temporal relation is t,, D t°~. 
V) It was raining(I). There was a strong 
wind(2). 
A~ DE COLING-92, NANTES, 23-28 ^OUT 1992 3 3 2 PROC. OF COLING-92, NANTBS, AUO. 23-28, 1992 
Tile eventualities of sentence (x) and (2) are: 
event(c1, rain), time(eht,,); 
event(e=, wind_stroug), time(e~,t0~); 
the temporal relation is t,, c~ t°~. 
V1) John sat down on a chair0}. Mary lied 
down on a sofa(2). 
The eventualities of sentence (1) and (2) are: 
event(el, sit(john, chair1)), time(el,t,,); 
event(e~, lie(Mary, sofa)), time(e~,t,~); 
the temporal relation is tst none to~ as these two 
eventualities are independent. 
3 Discourse Segments 
A discourse segment is a discourse object. It 
is represented by a discourse referent that can 
be used for later reference. In contrast to other 
discourse theories, segments ms dynamic struc- 
tures that help to define context interpretations 
are considered as real discourse objects. Thus in 
our approach we use segments as objects with 
properties that will be defined later. A text is 
represented by a segment and a segment supplies 
context information for the semantic interpreta- 
tion during discourse processing. 
Next segments will be defined as well as their 
construction and use in the semantic interpreta- 
tion. 
3.1 Kind of segments 
We distinguish two kinds of segments: basic and 
non-basic ones. 
A basic segment represents an eventuality plus 
some features, typically syntactic ones like tense 
and aspect (tile leaves of fig. 1). 
A nonbasic segment has one or more subseg- 
ments (basic or not) obeying to a set of temporal 
constraints and a set of features. Every nonbasic 
segment has a sort depending on the temporal 
constraints it imposes on its subsegments. 
Segment features are necessary for discourse 
reasoning. Some of them may be dropped after 
a closing but others have to remain until the dis- 
course is completely processed. The features we 
take into account in this paper are the following: 
• tense - The feature tense is needed for tem- 
poral anaphora resolution. 
• eventuality - The semantic representation 
of an eventuality is important for temporal 
anaphora resolution, for causal reasoning and 
other kinds of reasoning that depend on the kind 
of the eventuality. 
• eventuality time - This is the main issue 
ill the definition of a segment as the abducted 
relation between eventuality times determines 
tile segment structure's behavior. 
• discourse referents - for solving discourse 
reference. 
• subsegments - an ordered list containing all 
its subsegments. 
3.2 Sorts of segments 
Depending on the abducted temporal relation 
between eventualities in a discourse, the even- 
tualities are grouped into different sorts of seg- 
ments. Using the above mentioned five tempo- 
ral relations seven sorts of segments ~ can he de~ 
fined, e.g. 
1. basic - the minimal segment. 
2. none - this segment does not impose any 
restriction on tile temporal relation of its sub- 
segments. The discourse of example VI will be 
represented by this sort of segment. 
3. sequence -- the subsegments in the list of 
segments are temporally ordered, e.g. ex I (fig. 
l.a). 
4. fb - contains only two subsegments with the 
first one temporally situated after the second 
one, e.g. ex H (fig. 1.b). 
5. bk - has two subsegments with the first one 
temporally contained in tire second one, e.g.ex 
III. 
6. elab - has two subsegments with the first 
one temporally containing the second one, e.g. 
ex IV. 
7. over - every segment in the list of subseg- 
ments must temporally intersect a nonempty 
time interval, e.g. ex V. 
For each sort of segment it must be defined 
how to compute its features representing prop- 
erties from the features of its aubsegments. 
3.3 Properties of Segments 
Segments that can have a list of subsegments 
containing an unlimited number of segments are 
none, sequence sad over. These segments can 
be augmented during discourse processing. The 
features of these segments are the following: 
a none - The feature eventuality contains the 
set of all subsegments' eventualities, while the 
2The nantes of these tmgments ar~ abbreviationt of 
some rhetorical relatlona that impose the marne temporal 
cormtralnts. There abreviations should not be read as if 
they meant the same M the rhetorical relations. They 
jute mean that their sub~egments obey a particular tern- 
poral relation. 
AcrEs DE COLING-92, NANTES, 23-28 AOt~'r 1992 3 3 3 PROC. OF COLING-92, NANTES, Auo. 23-28, 1992 
le~lUenc~ 
¢venl(e3,~l(el,e2)) 
time(e3,t3), t.~lt li,t2q 
~me=~. An=t~rf 
fim~el.t !) tim~e2~t2~ tl<t2 
~m~sv. Av~cd. ~m~e~p. ApffiVcd. 
{a} 
fh 
©ven~© I ,buy~j,11mb I ) ) I 
\[ time(el,tl) 
Itease=~, An=perf.. 
¢vent(e 1 ,tmY(J,umlbl )) I ~v~/l¢(c2,k~o~(J,~a~b2)) I 
tln~el,tl) ~ime(e2,t2), ;2<11 
tense~p. Ap=ved. I~nse-pf, AP=verf. 
tb) 
Figure 1: Segmentation for ex. I and lI 
feature time contains the set of all subsegments' 
times. The tense feature refers to the tense of 
the last subsegment. 
• sequence - the feature eventuality is the 
composition of all the segments' eventualities 
obtained by the seq operator. The time feature 
interval is the time interval \[t,,~, t,,s \], where t, h 
is the initial point of the first segment time inter- 
val and t,,s is the final point of the last segment 
time interval. The tense feature is the tense of 
the last segment (fig. 1.a). This segment can 
be augmented by adding a new segment to the 
list of segments. In this case the features of the 
sequence segment have to be evaluated again s. 
• over- the feature eventuality is unknown, 
the feature time is the time interval of the inter- 
section of all time segments. The tense feature 
is the tense of its last subsegment. 
The segments that have two subsegments are 
fb, bk, and elab. These segments can only he 
augmented by the replacement of its second sub- 
segment by a new one obeying the same set of 
constraints. The replaced segment is the first 
subsegment of the new one. These segments 
have the same features of the first subsegment 
(fig. 1.b for a segment of sort fb). 
3.4 Discourse Segmentation 
Discourse segmentation will be done incremen- 
tally. A sentence will be represented by a seg- 
ment. The processed discourse will be repre- 
sented by a segment. 
3A notion similar to the sequence segntent is used in 
other theorl¢~ for discourse segmentation named in RST 
\[15\] u "narration", in Hobba \[9\] also As "narration" and 
in planning \[10\] as U~quence'. 
The steps for the discourse segmentation are: 
1. to obtain the first sentence segment. This 
segment will be the discourse segment. 
2. to obtain the segment representing the next 
discourse sentence. 
3. to insert the new segment in the discourse 
segment. This step will have the following sub- 
steps: 
(a) To compute the set of visible segments (i.e. 
the right subsegments of the discourse segment) 
from the discourse segment. This set of seg- 
ments can be ordered by some discourse pref- 
erence rule, e.g. we may prefer to continue the 
current segment or prefer to continue the first 
opened segment and close the other ones 4 . 
(b) to choose one segment sl from the set of 
visible segments. 
(c) to add the new sentence segment s2 to seg- 
ment s I by: 
• continuing the subsegment list of Sl if s2 
can satisfy the constraints of sl. 
• substituting sj by a new one as. sz con- 
tains s I as first subsegment and s2 as second 
subsegment in its subsegnlent list. The sort of 
segment s3 is one of the 6 nonbasic ones. 
(d) if it is not possible to add the new segment 
then choose another segment from the set of the 
visible ones, call it sl and try again going back 
to step 3c. 
4. go back to step 2 if there are more sentences 
to process in the discourse. 
3.5 Abduction of temporal relations 
The main process in discourse segmentation is to 
check for temporal relations between segments 
because this is the only criterion used for seg- 
mentation. 
For deciding how to link segment s2 given seg- 
ment sl, do: 
1. find the referent for s2. 
• if sl is going to be continued by s2, then 
the referent will be the last subsegment in the 
subsegment list of segment Sl. 
• if sl is going to be substituted by a new 
segment sa then the referent will be Sl. 
4The way segments are ordered will have conse- 
quenc~ on the discourse segmentation. In caAes where 
there are more then one possible segmentation preference 
will be given to the first one. 
ACTF.S DE COLING-92, NANTES, 23-28 ^OfYr 1992 3 3 4 PROC. OF COLING-92, NANTES. AUG. 23.28, 1992 
2. After obtaining a referent, abduct the tem- 
poral relation between s2 and the referent. 
3. After getting the relation between s2 and 
the referent, do: 
* if sl is to be continued by s2 then cheek 
if s2 satisfies the set of constraints of st. If so, 
update the st features if needed and repeat the 
procedure (update features in the parent node 
and check constraints) until the root segment or 
a node segment whose features don't need to be 
updated is reached. If this process terminates 
successfully then s2 can continue st. 
• if s2 is to be replaced by a new segment 
sa then compute the features of segment sa and 
check if the set of temporal constraints of the 
old parent of segment sl is satisfied. If so, up- 
date the old parent st features if needed and re- 
peat the procedure (update features in the par- 
ent node and check constraints) until the root 
or a node segment whose features don't need to 
be updated is reached. If this process ends sue- 
ceasfully then sa can replace st. 
Whenever a temporal entity in the form of 
a discourse referent is added to the discourse 
structure, the structure containing all temporal 
discourse referents as well as their temporal con- 
straints should be updated (fig. 4 for ex. of see. 
4). This way we can distinguish relations that 
are implied by the temporal system from those 
inferred using other knowledge sources, e.g. the 
constraints tl < t2, ta < t2 do not imply tl < ta, 
but satisfy it. 
In order to check for temporal constraints we 
do not only deal with constraints over tempo- 
ral intervals but use also world knowledge for 
abducting relations between eventualities that 
imply some temporal relation between them. In 
order to abduct a temporal relation it is not 
enough to block inconsistencies in the tempo- 
ral system. There should also be some kind of 
justification, like: 
* temporal - if the temporal system implies 
relation t0,@t,~ there is a justification to abduct 
tot@t,~, with @ being a temporal relation. 
• causal - if et can cause e2 then there is a 
justification to abduct t~ < t~. 
• contingent - Assuming an event ontology 
like that of Moens and Steedman \[17\] where 
eventualities have a tripartite structure with a 
preparatory phase, a culmination and a conse- 
quence state there are clauses stating what are 
the eventualities of the preparatory phase and 
those of the consequence state of an eventuality. 
- if el can be in the preparatory phase of e2 
there is a justification to abduct t~ C re2. 
- if el can be in the consequence state of e2 
there is a justification to abduct tet > re2. 
• particular shared knowledge about eventu- 
alities - having a temporal knowledge base con- 
cerning eventualities, general rules for eventual- 
tries can be stated, e.g. John usually drinks a 
cup of coffee before he catches the bus. 
- if e2 usually happens ~ et with ~ being a 
temporal relation, then there exists justification 
to assume te,t~tea. 
• Linguistic- verb tense and aspectual per- 
spective (AP) are the linguistic features ~ that 
contribute to the temporal anchorage of eventu- 
alities. There are rules that justify a temporal 
relation taking into account these features and 
the order of the sentences. They should be used 
as default rules, i.e. if it can not be found an- 
otber justification for a temporal anchoring then 
a linguistic justification should be used. The fol- 
lowing rules are used in the detailed example. 
if tense of el and of e2 is simple past (SP) 
with perfective AP tben there is justification for 
assuming t~ < re2. 
- if tense of el is SP with perfective AP and 
tense of e2 is SP with imperfeetive AP there 
exists justification to assume tea C it2. 
If a justification for the abduction of a par- 
ticular temporal relation is not a logical conse- 
quence of the knowledge base then the justifi- 
cation sbould be added to the set of conditions. 
e.g. if we abduct in the discourse "John fell from 
the balcony. Mary pushed hzm." that John fell 
because Mary had pushed him, we should add the 
clause "eause(et, e2)" in order to block the fu- 
ture inference tbat Mary was innocent in John's 
fall. In this example another interpretation, lin- 
guistically justified, could be t~l~t I < t~pu,h s. 
4 Detailed Example 
Consider the following discourse represented by 
the segment in fig. 3. 
Last month I boughl a house (sl). It had an 
aquartum (s~). Mary offered me a red fish (sa). 
John gave me his frog (st). My fish died yes- 
terday (ss). It stopped breathing (,~). It became 
blue (st). It went to the top of the aquarium (*s). 
SThese features may restrict the act of possible tem- 
poral relatiorta between two eventualities. 
8When there are more then one poulhle temporal 
relation a system mult ch~e one, but it ehould be able 
AcrEs DE COLING-92, NANTES, 23-28 nOr3T 1992 3 3 5 PROC. OF COL1NG-92, NANTES, AUG. 23-28, 1992 
(,) 
fo) 
(c) 
~) (c) 
(i) 
Figure 2: Segments while proce~ing ex. sec. 4. 
Figure 3: Segmentation for the ex. of see. 4. 
Figure 2.a represents the discourse segment 
after processing sentence st. The only way to 
incorporate the sentence s2's segment is by re- 
placing the current discourse structure by a new 
one (basic segments cannot be continued). Sl is 
the reference to anchor s2, the eventuality of s~ 
is a state, so the relation t~t C t,~ is abducted 
and the new segment is of sort bk (fig. 2.b). 
To insert Sa there are two visible segments: s2 
and bk. Segment bk is the reference for sa be- 
cause the eventuality of segment s2 is a state 
with an imperfective aspectnal perspective and 
there is no general knowledge about the even- 
tualities of s2 and sa allowing us to abduct a 
temporal relation between them. The features 
of the bk segment are the same as those of sl, so 
the abducted relation between those segments 
is t,~ < G~, so that the discourse structure is 
replaced by a new segment of sort sequence (fig. 
2.c). To insert segment s4 there are two visible 
segments, namely s3 and sequence. Using sa as 
reference, no temporal relation can be abducted. 
Thus none is abducted using some general rule 
saying "eventualities to give and ~o offer are of 
to backtrack to that choice point. 
IMt nlt~th yeetorday now 
Figure 4: Temporal anchor ofex. sec. 4. 
the same kind and if the subjects and objects 
are different and the recipient is the same it is 
not poesible to abduct a temporal relation be- 
tween them". So the segment s3 is replaced by 
a new segment of sort none (fig. 2.d) after the 
check to see if it is possible to abduct the re- 
lation t,b k < t,, in order to satisfy the con- 
straints of the sequence segment. The time of 
segment s5 is partially anchored in the time in- 
terval "yesterday" but it still must be anchored 
to a reference. Visible segments are s4, none 
and sequence. Considering that the abducted 
relation between s4 and s5 is t,4 < ~,~ and be- 
tween s3 and s~ is t,a <: t, 8 it is not possible to 
use s4 as referent because the constraints of seg- 
ment none were not satisfied. So the sequence 
segment is the only one left and it can be contin- 
ued by s5 (fig. 2.e) because its constraints are 
satisfied, thus te~ < tss,t,. < t. s are abducted. 
In order to insert s6, the visible segments are ss 
and sequence. Using s5 as referent, the relation 
t,, C 6, is abducted because stop breathing is 
in the preparatory phase of dying. Segment s5 
is replaced by a new segment of sort elab (fig. 
2.f). In order to insert segment sT, segment s6 is 
replaced by a new sequence segment because the 
relation t,~ < t, 7 is abducted (becoming blue is 
in the consequence state of stop breathing) and 
t, T C t,~ is abducted (becoming blue is in the 
preparatory ph~e of dying) (fig. 2.g). The in- 
sertion of ss leads to the final structure (fig. 3). 
ss cannot use s7 as reference because the rela- 
tion t,. C t,~ cannot be abducted. 
After processing the discourse, the temporal 
discourse referents and their relations reflect the 
structure of fig. 4, containing all the temporal 
relations that can be inferred from the text with- 
out making unjustified abductions. 
5 Conclusions 
Our segmentation method using abduction of 
temporal relations between eventualities as a 
ACIT~ DE COLING-92, NANTES, 23-28 AOt)r 1992 3 3 6 PROC. OF COLING-92, NANTEs, AUG. 23-28, 1992 
leading criterion for segmentation "/ has advan- 
tages over other currently used segmentation 
techniques. It uses a criterion whose precision 
and feasibility is supported by previous work 
in the area of temporal anaphora resolution 
\[19, 22, 18, 16, 4\]. Our discourse, segmentation 
provides the information necessary for tense in- 
terpretation (as required in \[22\]), and for con- 
straining further segmentation. Although Web~ 
her's temporal focus structure depends on the 
linguistic discourse structure \[7\], she doesn't ex- 
plain how tense interpretation contributes to the 
further development of tile linguistic structure. 
Other work tackling the interpretation of tense 
and aspect \[19, 18, 13, 4, 2, 16, 3\] does not intend 
to build up a discourse structure in the Grosz 
and Sidner's \[7\] sense. In \[13\] rbetorical rela- 
tions between some eventualities are build up, 
not discourse segments. Work in discourse seg- 
mentation based on plan recognition \[20\] does 
not provide the necessary information for tense 
interpretation either. Our structure intends to 
provide the context necessary for the interpre- 
tation of different kinds of anaphora. Our ap- 
proach enables us to compute both, the tempo- 
ral relations explicitly expressed in the discourse 
and those implied by the temporal structure 
of the discourse. The temporal coherence can 
be evaluated by looking at the final discourse 
structure, e.g. if a discourse is represented by 
a segment of sort none, no temporal relations 
between the eventualities described can be ob- 
tained and thus the discourse is not temporally 
coherent. Our approach does not prevent us 
from using complementary criteria for thc fur- 
ther segmentation of discourse, namely from us- 
ing other criteria such as clue words, spatial re- 
lations, and recognition of state elaboration. 
ACRES DE COL1NG-92, NANfES, 23-28 AOl3"r 1992 3 3 7 PROC. Or COLING-92, Natzrl.:s, AUG. 23-28, 1992 

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