Building Another Bridge over the Generation Gap 
Leo Wanner 
University of Stuttgart 
Linguistics Department 
Keplerstr. 17 
D-70174 Stuttgart, Germany 
leo@ims, uni-stuttgart, de 
Abstract 
In this paper, we address one of the central problems 
in text generation: the missing link ("the generation 
gap" in Meteer's terms) between the global discourse 
organization as often provided by text planning mod- 
ules and the linguistic realization of this organiza- 
tion. We argue that the link should be established 
by the lexical choice proces s using resources derived 
from Mel'~uk's Lezical Functions (LFs). In particular, 
we demonstrate that sequences of LFs may well serve 
as lexical discourse structure relations which link up to 
global discourse relations in the output of a Rhetorical 
Structure Theory style text planner. 
1 Introduction 
1.1 Identifying the Generation Gap 
In text generation, content selection and discourse or- 
ganization (i.e. the text planning tasks) have often 
been dealt with independently from the linguistic re- 
alization of the tezt plan- the information selected 
and structured by the text planning process (eft, e.g., 
\[McKeown and Swartout, 1987\]). However, a text 
planning process which does not take into account lin- 
guistic resources that are available to express a par- 
ticular meaning suffers from two major shortcomings: 
(i) it is not sensitive to variant discourse organiza- 
tions at the sentence level; and (ii) it cannot guar- 
antee that its text plan is always verbalizable by the 
linguistic module. With other words, there is a dis- 
crepancy (a "generation gap" in Meteer's terms) be- 
tween a text plan that is not tailored to linguistic 
resources and the input as required by the linguistic 
realization module. For extensive examples that illus- 
trate this, see, especially Meteer's work \[Mercer, 1991\] 
and \[Mercer, 1992\], but also, e.g., \[Rubinoff, 1992, 
Vander Linden et al., 1992\]. In our work, we have 
found that especially lexical phenomena (such as lex- 
ical cooccurrence and lexical semantics) play an im- 
portant role in discourse organization at the level of 
sentence planning, which is still one of the unsolved 
problems in text planning, cf. \[Hovy, 1991\]. Consider 
the following example: 
(1) a. Opa schofl auff den Einbrecher, der 
nun tot ist lit. 'Grandpa shot at the burglax who 
is now dead'. 
The interpretation of (la) does not necessarily suggest 
that burglar's death is in consequence of grandpa's 
shooting. To communicate this relation, (lb) is more 
appropriate: 
(1) b. Opa sehofl au\] den Einbrecher und 
tJtete ihn/ verwundete ihn tJdlich lit. 'Grandpa shot at the burglar and 
killed him/wounded him deadly'. 
However, despite an analogous syntactic construction, 
(2) suggests the interpretation that grandpa's being 
well is in consequence of doctor's curing: 
(2) Der Arzt heilte Opa, der nun ganz gesund ist 
\].it. 'The doctor cured grandpa, who is now completely well'. 
That is, the consideration of lexical phenomena 
helps to resolve the discrepancy between discourse 
structure relations (as, e.g., given by Rhetorical Struc- 
ture Theory (RST) \[Mann and Thompson, 1987\]) and 
their linguistic realization. Therefore, a text genera- 
tor has to provide an organization of lexical resources 
which makes explicit the rhetorical potential of lexical 
phenomena and which allows for the purposeful choice 
of these phenomena. 
In principle, three ways seem to be feasible for how 
to proceed: (i) to broaden the task of text planning by 
lexical choice (eft, e.g., \[Mercer, 1992\]), (ii) to realize 
an interaction link between the linguistic realization 
module and text planning (eft, e.g., \[Rubinoff, 1992\]), 
or Off) to broaden the task of linguistic realization in 
order to deal with a final, lexically guided determina- 
tion of a discourse organization that has been prede- 
termined by text planning. 
137 
7th International Generation Workshop * Kennebunkport, Maine • June 21-24, 1994 
(R1/coNsCqUZRCZ 
:ac¢ion (SROOT / SITUATION 
:actor (GRAmDFATXER/ PERSON 
: in-focus '+) 
: actee (BURGLAR/ PERSON) 
: obligal;ory-roles ( : acl;or, : acl;ee, 
: sitllat ion ) ) 
:consequence (BZIIIG-DEAD / SITUATION 
:carrier (BURGLAR/ PERSON) 
: obligatory-roles ( : carrier, 
: situation))) 
Figure 1: A simplified text plan for sentences with the 
meaning 'Grandfather shoots at a burglar; in conse- 
quence, the burglar is dead.' 
" spelling: ~schiellen ~ "~ 1 
/ \[sp~tUag: "Op-"l \[actor: I~LEX-OPAlperson: + \] 
/ Lsin~'.da~: + J ,, | \[spelling: "Zlnbrecher \] 
\[ actee: LEX-EINBRECHER\[person: + | 
/ L n°un: + J 
Ltheme: la\] • 
"~'0 .... -~pe\].ling: ~verwunden ~\] 
,. ..... Lv~b: ÷ \] ', 
m ......... A ..... \[spe\].ling: "t~illch'\] 
................ \[qutdJty: ÷ \] 
\[spellihg: "Opa '~\] 
actor: LEX-OPA.I person: ÷ \[ Lsi"v ai~'- + ,i 
\[spelling: "Einbrecher"\] 
actee: LEX-EINBRECHERI person: ÷ | Lsms~.l~: + J 
.theme: \[\] .~ 
In our work, we follow (iii). This is, in order to 
keep s separate level of discourse organization that is 
realized independently from linguistic resources and 
to make use of the potential of linguistic resources to 
guide the discourse organization at a more detailed 
level than conventional text planning is able to do. In 
contrast to, e.g., \[Fawcett et al., 1992\] and \[Elhadad 
and Robin, 1_992\], who deal with lexically biased sen- 
tence organization within the grammar, we take a rad- 
ically lexicalist position (similar to that of \[McDonald, 
1991\]) in that we assume that lexicalization has to 
take place before grammatical realization; and that it 
is the words chosen, which dictate the possible syn- 
tactic realizations of a content to be communicated. 
More precisely, we propose a two level discourse or- 
ganization where the first level is provided by an RST 
style text planner and the second level by a separate 
lexical choice module. Then, the discourse organiza- 
tion of a text is done in two steps: in the first step, 
the text planner predetermines the discourse structure 
relations and selects the content; in the second step, 
the lexical choice module provides, in accordance with 
linguistic constraints, a finer specification and the re- 
alization of these relations and tailors their content to 
linguistic realization. 
1.2 The Framework 
The computational framework in which our lexical 
choice model has partially been implemented is the 
systemic text generator KOMET \[Bateman et al., 1991\]. 
One source of constraints for the first level text orga- 
nization comes in KOMET from an RST-based planner, z 
The output of this planner is a collection of case frames 
with RST relations holding between them as shown in 
Figure 1. 
Starting from such a text plan, the lexical choice 
1 Recent developments of this planner are described in \[Hovy 
et al., 1992\]. 
Figure 2: PGS structure for the sentence Opa schofl 
auf den Einbrecher und verwundete ihn tSdlich lit 
'Grandpa shot at the burglax and wounded him deadly' 
module has to construct an input to the grammar 
(in KOMET m a systemic grammar of German \[Te- 
ich, 1992\]). This input contains the lexicalization of 
the text plan content and constraints for its syntactic 
realization. Figure 2 shows such an input (called Par- 
tial Grammatical Structure (P6S)) 2 encoded in terms 
of Typed Feature Structures (TFSs) \[Emele, 1989\]. This 
input has been derived from the text plan in Figure 1.s 
In what follows, we present an ongoing attempt to 
define a general model of lexical resources which would 
provide the missing link between the global discourse 
organization as given by discourse structure relations 
in the text plan and its linguistic realization. In partic- 
ular, we show how Mel'~uk's Lezical Functions (LFS) 
(cf., e.g., \[Mel'~uk and Polgu~re, 1987\]) can be used 
to compile such a general model. After the introduc- 
tion of the general principles that underly our organi- 
zation of lexical resources, we focus on LF sequences. 
We demonstrate that they may well function as lex- 
ically biased discourse structure relations at the in- 
trasentential level; and, thus, be interpreted as a finer 
specification of global discourse structure relations. 
2A oos corresponds, roughly speaking, to the Partial Sur- 
face Functional Description (PSFD) specification in the COMEr 
system \[MeKeown et aL, 1991\]. 
SThe current implementations of the lexical choice module 
and the grammar still require an additional mapping between 
the pGs and the input ax taken by the grammar. Since this 
mapping is, however, purely due to implementational details, it 
is of no interest here. 
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7th International Generation Workshop • Kennebunkport, Maine • June 21-24, 1994 
2 Overview of the Lexical Re- 
sources Organization 
While designing a lexical choice model, it is important 
to note that (a) lexical phenomena and, therefore, also 
lexical choices that are available to communicate a 
specific meaning are dependent on this meaning; and 
(b) lexical phenomena are involved at different stages 
of the utterance construction process. Thus, the pro- 
cess of SHOOTING has another lexiealization potential 
than that of CURINC; and the choice of fierce in The 
terrorists put up fierce resistance is done at another 
stage than the choice of put up. This calls for: 
• a (fairly) excaustive coverage of the lexical po- 
tential of each semantic entity (such as process, 
event, etc.); 
• clustering of lexical resources locally to semantic 
units of a certain size; 
• a multilayered organization of lexical resources 
(and therefore also a multilayered lexical choice 
process). 
In our model, the resources are provided mainly by 
Lezical Functions as introduced in the Meaning Text 
Theory (MTT). They are clustered locally to situations 
and organized into four layers. 
2.1 Using Lexical Functions as the 
Source of Lexical Resources 
Most excaustively, hxical phenomena have been de- 
scribed in MTT by £exical Functions. 
Roughly speaking, an LF f is a standard semantico- 
lexical relation which holds between a lexeme L, (the 
keyword of f) and a set of lexemes f(L) (the value of 
If). Consider the following examples: 
So: 'situation' 
Sl : 'actor' 
Sin~tr: 'instrttment ' 
Ao: 'situational adj? 
V0: 'action' 
A.nti: 'antonym' 
Gener: 'hyperonyra' 
Magn: 'intense(ly)' 
Incep: 'beginning' 
Operl: 'perform' 
Prepar: 'prepare' 
Prox: 'be about to' 
So(to shoot) = shot 
Sl(tO ShOOt) = 9=nman, marksman 
Sinstr(to shoot) = r/fie, ann .... 
Ao(death) = mortal 
Vo(,ho0 = \[to\] ~hoot, \[to\] ~,, 
Anti(to hit) = \[to\] miss 
Gener( rifl,) = fire-arm 
Magn( injury) = ,e~ere 
mcep(com=) = \[to\] tap,~ 
It,to\] (corn,0 
Operl(shot) = \[to\] take 
(a shot) 
Prepar(to ,hoot) = \[to\] charge 
Prox(to shoot) = \[to\] aim at 
Mel'~uk distinguishes about sixty simple LFS of the 
above kind. Simple LFs can further be composed with 
each other; the meaning of such complex LFs is, as a 
rule, a composition of the meanings of the participat- 
ing LFS. Thus, AntiMagn means 'slightly' (e.g., An- 
tiMagn(injury) = minor); and IncepOperl 'start 
performing' (e.g., IncepOper1(shooting) = \[to\] start 
(shooting)). If several (simple or complex) LFs com- 
pose a phrase or a clause (as, e.g., AntiMagn and So 
compose minor injury), we separate these LFs by a 'o' 
sign. 
In addition to complex LFs, we introduce LF 
sequences -- pairs of LFS (denoted LF1 A LFz), 
whose values cooccur at the syntagmatic axis. For 
example, Prepar(shooting) A Vo(shooting) with 
Prepar(shooting) = \[to\] charge (the gun) and 
Vo(shooting) = \[to\] shoot in He charged the gun and 
shot. LF sequences are directed, i.e. LFx A LF2 
LF2 A LFx. Consider, e.g., Prepar A V0, which is 
an instantiation of a TEMPORAL SEQUENCE relation 
with the first LF expression preceding the second on 
the temporal axis, and V0 A Prepar as in He shot 
after charging the gun, where the first LF expression 
succeeds the second. Moreover, the existence of LFx 
A LFz for an entity in a language does not mean that 
LF2 A LFz is also available (consider, e.g., the follow- 
ing instantiation of A0 A Magn o So: She is beautiful 
-- a real beauty and Magn o So A A0: *She is a real 
beauty -- beautiful. Therefore, in LF sequences, one 
argument is the 'hub' m the point of departure (or 
the expanded LF) and the other argument is the 'hub 
expander'. 
LFs and LF sequences (henceforth, both are referred 
to as 'LFS') have well-defined syntactic realizations. 
For example, Operz is defined as taking the ACTOR 
as grammatical'subject and the keyword of the sit- 
uation as direct object. The LF sequence Operl A 
Magn o So -- when applied to SHOOTING m can be 
realized as a paratactic complex clause (of. Grandpa 
took a shot at the burglar; it was a good shot.~ or as 
a simple clause (eft Grandpa took a good shot at the 
burglar), but not, e.g., as a hypotactic complex clause 
(eft *Grandpa took a shot at the burglar, which was a 
good shot/one). And so on. 
2.2 Compiling Lexical Resources Lo- 
cally to Situations 
In accordance with \[Apresjan, 1974\], a situation is 
comprehensively defined by: 1. a predicate; 2. the 
constituents (i.e. potential participants and circum- 
stantials) of this predicate; 3. the relations between 
different constituents as well as between constituents 
and the predicate (these relations are usually given in 
terms of actions, states, etc.); 4. the features of the 
predicate, constituents, and the above actions and re- 
lations. 
The lexical resources of a situation (the Lezicaliza- 
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7th International Generation Workshop • Kennebunkport, Maine • June 21-24, 1994 
tion Cluster in terms of \[McDonald, 1991\]) are then 
given by all possible wordings of the entities this sit- 
uation is defined by. For example, SHOOTING is a sit- 
uation with the predicate named shooting, shot, etc.; 
with the participants ACTOR (named gunman, marks- 
man, etc.) and ACTEE (named target, victim, ...); 
with the obligatory circumstantials INSTRUMENT (r/- 
fie, gun, etc.) and MEANS (bullet), and optional cir- 
cumstantials LOCATION, TIME, etc., which do not have 
a situation-specific lexicalization. Among the ACTOR'S 
actions are shoot, fire, kill, wound, etc. The lexicaliza- 
tions of the relations between ACTOR and the pred- 
icate include, e.g., the performance of the situation 
(fire (a shot), take (a shot), etc.); those between AC- 
TOR and ACTEE -- aim (at), hit, etc. The features of 
ACTOR include skilled, good, lousy, .... And so on. 
The various LFs -- with the label of a situation 
as keyword -- supply us with all lexical expressions 
that communicate the entities this situation is defined 
by; see the list above for examples with respect to 
SHOOTING .4 
2.3 Organization of Lexical Resources 
as a Multilayered Model 
As stated above, we organize situation-specific lexi- 
cat resources that are provided by the LF information 
into four layers. The layers are called Intersituational 
Layer, Intrasituational Layer, Constituent Layer, and 
Lexeme Layer, respectively. The first three layers are 
motivated by the semantic constituency of expressions 
as provided by LFS: 1. two predicate structures (as 
given by LF sequences) are dealt with at the intersitu- 
ational layer; 2. predicate structures (as given, e.g., by 
the Operx function) -- at the intrasituational layer; 
and 3. predicate constituent structures (as given, e.g., 
by the St function) -- at the constituent layer. At 
the lexeme layer, the LFS are unified with concrete 
lexemes, s 
LFs correspond to knowledge at varying levels of ab- 
straction (see the discussion in \[Wanner and Bateman, 
1990\]); therefore, we organize them coherently at each 
of the first three layers in network form in terms of the 
functional, semantic, and syntactic features that they 
represent (cf. also \[Wanner, 1992\]). For example, the 
Operl function is decomposed as {actor.realization, 
situation-oriented, actor-salience, ...}. Within this 
organization, the most general structures provide the 
representation of lexical semantics and the most deli- 
4Evidently, semantically related situations have intersecting 
lexicalization clusters. In order to achieve an efficient organi- 
zation of the overall lexical resources, we use inheritance tech- 
niques, cf. \[Wanner, 1992\]. 
SObviously, our layers are in analogy to the "ranks" in sys- 
temic functional grammars, cf. \[Halliday, 1985\]. 
cate ones -- lexicalization. 
A PGS is then built up b\] the lexical choice process 
by a successive traversal of the layers starting from the 
intersituational layer. How this is done is described 
in detail in \[Wanner, 1992\]. Here we focus on the 
organization of the layers and on the interface between 
the output of the text planning module and lexical 
resources. 
In what follows, each layer is described briefly 
(starting with the lexeme layer). Note that this de- 
scription is necessarily extremely oversimplified; it 
largely abstracts from the actual representation in a 
systemic functional framework. 
Lexeme Layer. The lexeme layer contains LF - LF 
value pairs. Consider selected entries for Operl and 
$1 in the lexicalization cluster of SHOOTING: 
L.o~: + J 
Constituent Layer. The constituent layer contains 
feature specifications for the realization of predicate 
constituents and their attributes, s For example, to 
name the ACTOR in SHOOTING, his/her proper name; 
a situation non-specific realization, e.g., his/her func- 
tion in relation to other entities in the knowledge 
base (e.g., grandfather); the situation-specific and con- 
textually neutral lexeme marksman; or a negatively 
loaded situation-related expression such as murderer, 
killer, etc. can be chosen. Consider a sample frag- 
ment for the realization of the ACTOR in SHOOTING in 
network form: 
actor- - situation-spectr~- 1 ne 1o" g.o ! 
mg -Io • . .o • 
The labels in Times Roman are network features in 
the corresponding fragment; those in italics are point- 
ers to "external features" that provide a more detailed 
specification of the feature they are associated with. 
"External" means 'at other layers of the same clus- 
ter' or 'in clusters of other situations'. Clusters of 
other situations are identified by labels in small cap- 
itals (preceded by an uparrow). The labels in bold 
denote roles which are introduced into the PGS when 
6To keep the presentation simple, we do not discuss the at- 
tributes here. 
140 
7th International Generation Workshop • Kennebunkport, Maine • June 21-24, 1994 
the corresponding feature is chosen during the network 
traversal. 
The fragment shows that there are pointers to fea- 
tures in. other lexicalization clusters and to features at 
lower layers of the same lexicalization cluster (of. the 
pointer to S1 at the lexeme layer). 
Intrasituational Layer. The intrasituational layer 
contains feature specifications for the realization of 
predicates and their attributes. For example, to ex- 
press ACTOR's performance of SHOOTING, e.g., V0 ( 
\[tO\] shoot), Operl (\[to\] take a shot, \[to\] fire a shot), 
etc. are available; but not, e.g., Oper2 (*\[to\] get a 
shot as in * The burglar got a shot from grandpa). In 
our functional interpretation, the difference between 
\[to\] shoot and the other options is that it is 'action- 
oriented', while the others are 'situation-oriented'. If 
\[to\] get a shot were grammatical, it would emphasize 
'actee-salience' (as \[to\] be shot does), while the other 
options mentioned emphasize 'actor-salience'. In net- 
work form, this looks as follows (see \[Wanner and 
Bateman, 1990\] for a detailed discussion): 
axis: the 'causation' and the 'initial' phases precede 
the 'continuing' and the 'final' phases; the 'continu- 
ing' phase precedes the 'final' phase, etc. Further, the 
phases 'initial' and 'final' are related to SHOOTING it- 
self. 
As global, for example, the different PREPARA- 
TION actions (PREPARATION / (verbalized, e.g., by \[to\] 
charge the gun) and PREPARATION// (verbalized, e.g., 
by \[to\] cock the trigger) precede SHOOTING; ACTOR's 
and ACTEE'S actions that follow SHOOTING as well as 
the 'resulting state' of ACTEE succeed it. In the follow- 
ing fragment, only the 'global' organization is partially 
illustrated: [ 
- gL tazce, aag... 
- gl. succ.eeainl 
..° 
eausatioa-~lt performzaee: 2:" 
F ~or-saue~| I~':. ,ao~-re,aizaao,\[ 
• . I I Iolt~: artee-realizationl r 
acdon-on~ted 1 " 
act ~ro~nan ~ actee-salience Pen' 4 
"'\[~,~*-,: oe,, \] L x 
/ / ,=,,+.,+:,<<,+,. j 
Note the difference between PGS (i.e. grammatical) 
roles and situation roles: the feature set { . .. actor's 
performance, situation-oriented, actor-salience} trig- 
gers the realization of the label of the situation (So) 
in the grammar as ACTEE; and the situational role 
ACTEE -- aS GOAL. 
3 Using Lexical Resources for 
Discourse Organization 
To tailor RST relations in the text plan to lexical re- 
sources, the lexical choice process must know (i) how, 
e.g., RST relations are specified in more detail by lex- 
ical resources and (it) what the possible linguistic re- 
alizations of these finer specifications are. Since LFs 
have well-defined semantic, lexical and syntactic real- 
izations, (it) is provided by our representation. In this 
section, we address (i); and more precisely: 
• how discourse structure relations are defined in 
lexical resources, 
Intersituatlonal Layer. The intersituational layer 
contains feature specifications for the realization of re- 
lations between predicates. For example, of the CAU- 
SATION relation between ACTOR's performance of the 
situation SHOOTING (verbalized, e.g., by \[to\] shoot or 
by \[to\] take a shot) and the state of ACTEE after the 
situation (\[to\] be dead); of the CONDITION relation be- 
tween ACTOR's preparing the INSTRUMENT for func- 
tioning and ACTOR's performing of the situation (as, 
e.g., instantiated by \[to\] charge and \[to \] shoot; etc. In 
the following fragment, we present the specification of 
the TEMPORAL SEQUENCE relation taking SHOOTING 
as reference time. We represent the 'internal' temporal 
and the 'global' temporal organization of SHOOTING. 
As 'internal', we present its phases at the temporal 
• how these relations are linked up with the rela- 
tions specified in the text plan. 
3.1 Towards Lexical Discourse Struc- 
ture Relations 
For the definition of discourse structure relations in 
lexical resources (henceforth lexical discourse struc- 
ture relations), we use LF sequences. As stated above, 
LF sequences (and LFs in general) are organized in 
terms of their semantic, functional, and syntactic fea- 
tures. 
The expression of the same content via different 
syntactic patterns is an important task in sentence 
planning (eft, e.g., \[Meteer, 1992, Vander Linden et 
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7th International Generation Workshop • Kennebunkport, Maine • June 21-24, 1994 
LF sequence I Function 
Operx A Operl insisting restatement 
Vo A GenerS0 generalizing, restatement, 
clarifying restatement, 
class-referencing 
V0 A NOT AntiVo contrastive restatement 
Ao A Magn o So attributive refinement, 
intensifying restatement 
Operl ^ Magn o So attributive refinement 
Caus A Vo causal enhancement, 
justification, 
external causation 
SinJtr A Vo instrumental enhancement, 
instrumental action 
'Prepa~ J A Operx temporal sequence, 
condit ion 
Prox A Vo i temporal sequence 
Result2 ^ V0 temporal sequence, 
explanation 
Table 1: Functional content of some LF sequences 
al., 1992, Scott and de Souza, 1991\]. 7 But it is the 
functional content we associate with each LF sequence, 
which links t_he lexical resources up to global discourse 
structure relations. 
Table 1 shows the functional content we associate 
with some LF sequences, s For a detailed discus- 
sion of how the functional content of LF sequences is 
used to define and to structure lexical discourse rela- 
tions following Halliday's organization of intrasenten- 
tial logico-semantic relations (cf. \[Halliday, 1985\]), see 
\[Wanner, 1994\]. 
. Caus2Operl A (Magn o) SynV0 (as in She 
commands respect -- I (really) think highly of 
her); 
2. V0 ^ because of o SUBJECT (as in I respect 
her because off her work); 
3. CaussOperx (as in Her work makes me respect 
her). 
The functional content of these realizations is more 
detailed than that of VOLITIONAL CAUSE. For exam- 
ple: 1. communicates an active role of the CAUSER; 2. 
-- the active role of the ACTOR, and 3. -- the active 
role of the SUBJECT. Therefore, they not only realize 
VOLITIONAL CAUSE, but also specify it in more detail. 
To summarize, the search for a lexical discourse 
structure relation is done in accordance with the func- 
tional content, the communicative intention of the 
speaker, and the contents of the arguments of the RST 
relation considered. If the RST relation connects un- 
related case frames 1° (as, e.g., EVIDENCE in In win- 
ter, the days are short. It is getting light late and 
early dark) these case frames are realized indepen- 
dently without being connected by a lexical discourse 
structure relation. If the case frames are related, the 
following three variations are possible: 
(i) An RST relation coincides, in general, with the func- 
tional content of a lexical discourse structure relation; 
as, e.g., VOLITIONAL CAUSE in the following rudimen- 
tary text plan for the 'RESPECT' examples given above 
does: 
3.2 Establishing the Initial Link 
The first and the most important task in tailoring the 
text plan to linguistic resources is to find lexical dis- 
course structure relations that correspond to RST re- 
lations specified in the text plan. 
To illustrate this task, let us consider the RST 
relation VOLITIONAL CAUSE as it holds between 
the situation's causation and the situation itself. 
The corresponding function in our model is causal 
enhancement. Apart from the LF sequence Caus A 
V0 given in the table above (in the cluster of RESPECT, 
instantiated, e.g., by An old tradition requires children 
4o show respect for their parents), at least the following 
three also function as causal enhancement: 9 
rClosely related to this topic is the task of 'compacting' the 
information to be corv_mtmJcated' (sometimes called 'aggrega- 
tlon' \[Hovy, 1993\]). 
STl~s is not to say that these functions are the only ones 
that are possible. 
9 The Syn function provides a synonym expression; the Caus 
indices '2' and '3' stand for 'causation by ACTEE' and 'causation 
by the SUBJECT', respectively. 
(R2/V0LITIONAL CAUSE 
:cause (NORK/ SITUATION 
: actor (PERSON 
: sex female 
:name #unknoen#) 
: obligatory-roles (: actor) ) 
:action (RESPECT/ SITUATION 
: senser (PERSON 
: speaker +) 
: phenomenon (PERSON 
:sex female 
:name #unknown#) 
: obligatory-roles ( : senser, 
:phenomenon, : situation))) 
If so, the subclassification of the lexical relation deter- 
mines its final realization. 
(it) An RST relation instantiation subsumes several 
distinct classes of lexical discourse structure relations; 
as, e.g., the instantiation of the RST relation CON- 
TRAST in the text plan below (this plan is also highly 
simplified): 
1°Case frames are considered to be unrelated if between them 
or one of their roles no identity, is-a, causer, location, etc. re- 
lation holds. 
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7th International Generation Workshop • Kennebunkport, Maine • June 21-24, 1994 
(R2/C0gTRAST :action1 (OCCUPATION/ SITUATION 
:actor Roman/ nation 
:actee Gaul/ state 
: obligatory-roles (:actor, 
: actee, :situation)) 
:action2 (OCCUPATI01~/ SITUATION 
:actor Roman/ nation 
:actee (village /location : part-of: Gaul 
:negation +) 
: obligatory-roles (: actor, : actee, : situation)) 
This text plan may be realized either as contrastive 
claxification: Gaul is entirely occupied by the Ro- 
mans; well, not entirely.., one small village still holds 
out.; or as contrastive enhancement: Gaul is almost 
entirely occupied by the Romans; but one small vil- 
lage still holds out, In this case, a sufficiently general 
lexical discourse structure relation which subsumes 
both contrastive clarification and contrastive 
enhancement is chosen. 
(iii) An RUT relation is not captured by our taxonomy 
(as, e.g., CONCESSION). Then, the corresponding case 
frames are treated as unrelated (see above). 
4 Related Work 
One of the first proposals for how to take linguistic 
resources into account during the process of text plan- 
ning was Danlos' Discourse Grammar \[Danlos, 1987\], 
where acceptable clause pattern sequences were pre- 
sented explicitly. The basic difference between Dan- 
los' work and ours is that in the Discourse Gram- 
mar, clause pattern sequences are represented as con- 
crete valency schemata while in our model, they are 
represented as functional distinctions that encode se- 
quences of LFS. ~S a result, we do not face the prob- 
lem of being restricted to a concrete small domain as 
Danlos does. 
Meteer's text planner \[Meteer, 1992\] is another pro- 
posal for the organization of lexical resources that 
incorporate lexically biased discourse relations. But 
while we argue that lexically biased discourse rela- 
tions are to be realized by a functionally motivated 
lexical choice model, Meteer sugggests a single struc- 
turally motivated model for text planning, which also 
subsumes lexical choice. 
Further, for example, \[Rubinoff, 1992\] ensures the 
expressability of discourse relations provided by a con- 
ventional text planner by annotating linguistic struc- 
tures. Dob~ and Novak \[Dob~ and Novak, 1992\] use 
RST structure relations and the Ted Structure repre- 
sentation proposed by Meteer in parallel: via RST rela- 
tions, the content selection and discourse organization 
is done; the representation of the arguments of the RST 
relations chosen in terms of Tezt Structures ensures 
the linguistic realization and provides constraints for 
the guidance of the process of content selection and 
discourse organization. \[Vander Linden et at., 1992\] 
suggest subclausal RUT relations for sentence organi- 
zation. Neither Dob~ and Novak nor Vander Linden 
et al. address lexical phenomena explicitly, however. 
Elhadad's proposal \[Elhadad, 1992\] to use Topoi 
(i.e. inference rules that encode relations between 
propositions incorporating lexical material) as dis- 
course structure relations is aimed at exploiting lex- 
ical phenomena for discourse organization. Elhadad 
focuses, however, on the 'argumentative potential' of 
lexical items rather than on a comprehensive model of 
lexical resources. 
5 Concluding Remarks 
In this paper, we argued that in order to build up 
the link between discourse structure relations in the 
text plan and linguistic resources, it is useful to dis- 
tinguish between two levels of discourse organization: 
a global discourse organization, which is not affected 
by linguistic means; and a finer discourse organiza- 
tion, which is clone in accordance with the linguistic 
materiM that is available for the meaning communi- 
cated. We reported on an ongoing attempt to define 
a situation-specific, multilayered model of lexical re- 
sources that is based on Mel'~uk's Lezical Functions. 
We have shown that the lexical phenomena repre- 
sented at the most global layer of this model are suit- 
able to serve as lexically biased discourse relations, 
and that these relations can be tailored to relations as 
specified in the output of an RST style text planner. 
The following distinctive features characterize the 
model proposed: 
• it makes sure that all relations defined are ex- 
pressable in language, 
* it allows for a realization of lexical relations as in- 
traclausal relations between discourse segments, 
• it is sensitive to lexical and syntactic variations 
during the realization of discourse relations. 
Although we decompose the LF information and re- 
present it in a systemic framework, we think that we 
have shown that LFS as introduced in MTT can be used 
as constraints not only at the level of words (as argued 
in \[Meteer, 1991, p. 302\]). However, it should also be 
noted that the proposal we suggest works only if the 
arguments of a discourse relation communicate infor- 
mation on the same situation or on related situations. 
Acknowledgements 
Many thanks are due to John Bateman for reading the 
various versions of this paper and for the long (phone) 
143 
7th International Generation Workshop • Kennebunkport, Maine * June 21-24, 1994 
discussions about lexical phenomena in discourse. I 
am also very grateful to Elisabeth Maier and Owen 
Rambow for valuable comments on an earlier draft. 

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