Discovery and Format 
of Input Structures for Tactical Generation 
Mark Seligman 
ATR Interpreting Telecommunications Res. Labs. 
2-2 Hikaridai, Seika-cho, Soraku-gun, Kyoto 619-02 Japan 
emaih seligman@itl.atr.co.jp 
ABSTRACT 
We are concerned with the proper form for the structures 
which serve as the output of strategic generation and the 
input to tactical generation. How should the elements of this 
structure be discovered and how they should be arranged? In 
both respects, our proposals will contrast with those of 
Rhetorical Structure Theory \[Mann and Thompson\]. Major 
components of the structure are discourse relations and 
"rhetorical types" (pragmatic objects similar to speech acts) 
to be discovered via paraphrase relations. We describe a 
discovery procedure and compare it with similar proposals 
by Ivir et al and Knott and Dale. We then argue that these and 
other components, once identified, should be arranged in 
lattices rather than strictly in hierarchies. As illustration, 
we list the discourse relations and rhetorical types needed to 
generate a paragraph-length discourse studied by Mann and 
Thompson. We present a lattice composed of these, and give 
one example of an alternate version of the discourse which 
could be generated from it. 
INTRODUCTION 
Our concern here is with the proper composition and form of 
the structures which serve as the output of strategic 
generation and the input to tactical generation -- that is, as 
the interface between these processes. We take discourse 
relations (essentially semantic recta-relations) and 
"rhetorical types" (pragmatic objects similar to speech acts) 
to be major elements of tactical input. Our brief is that these 
should be discovered via judgments of synonymy, and 
should be arranged in networks or lattices rather than 
exclusively in hierarchies. 
Section 1 motivates and describes a paraphrase-oriented 
discovery method based on a suggestion by \[Ivir et al 1973\] 
and compares it with a similar proposal \[Knott and Dale 
1992\]. Section 2 discusses rhetorical types and their 
discovery. Section 3 compares the paraphrase-based point of 
view with that of theories in the style of RST which 
concentrate on elucidating the speaker's underlying 
rhetorical goals. A general theme will be that much 
information concerning rhetorical goals and strategy should 
remain beyond the view of tactical generation programs. 
Section 4 defends the departure from strict hierarchy in our 
discourse networks. Finally, section 5 proposes a complete 
lattice as tactical generation input for a paragraph-length 
discourse studied by \[Mann and Thompson 1983, 1984, 
1985\] and gives one example of an alternate version which 
could be generated. The discourse relations and rhetorical 
types we feel are needed to generate this discourse are listed 
in an Appendix I. Each is accompanied by several possible 
expressive structures. 
We will be describing no particular programs or processes. 
We are concerned with strategic programs only in 
attempting to influence their final output. Concerning 
tactical generation procedures suitable for expressing the 
lattice of Section 5, see \[Seligman 1991\] . 
1. PARAPHRASE-BASED DISCOVERY 
OF DISCOURSE RELATIONS 
We believe that "discourse relations" (essentially, semantic 
recta-relations) and "'rhetorical types" (roughly, speech acts)1 
are major elements of an appropriate input structure for 
tactical generation, and that both sorts of elements can be 
identified by analyzing their surface linguistic expressions. 
\[Seligman 91\] presented a discovery methodology based on 
synonymy judgments, one intended to refine and make more 
explicit an earlier proposal appearing in \[Ivir et al 1973\]. 
Recently, an independent but quite similar suggestion by 
\[Knott and Dale 1992\] has introduced some further 
worthwhile refinements. 
Let us put forth the view of tactical generation which 
motivates this opinion. We can then go on to compare the 
three procedures. 
We take it as fairly uneontroversial that the role of a tactical 
generation program is to express certain preselected 
semantic and pragmatic structures. The strategic component 
has decided that certain meanings and functions are to be 
expressed; and the tactical component's assignment is to 
select any combination of expressors which preserve that 
meaning and function. Strategy has decided what to say; 
tactics must choose a way of saying it. 
But certain implications of this tactical assignment -- 
"select expressors so as to preserve meaning and function" -- 
need clarification. 
One implication, we think, is that a semantic or pragmatic 
symbol at the tactical level should be considered by its very 
definition a shorthand for a semantic or pragmatic 
equivalence relation: a way of capturing what semantic or 
pragmatic characteristics certain surface expressions have in 
common. And so a tactical generator's task with respect to 
such a symbol, stated a bit more carefully, is to choose 
among expressions which have been previously judged 
semantically or pragmatically equivalent in context and 
grouped under that symbol for just that reason. (And, since 
there will be many symbols, each with several possible 
structures, it must also make sure that the respective 
expressors are compatible.) 
From this reconceptualization or redefinition of the tactical 
role, it follows directly that a procedure for discovering 
semantic or pragmatic symbols at the tactical level should 
depend on speakers' semantic and pragmatic equivalence 
judgments. 
A commitment to restrict the tactical role to making 
expressive selections and checking expressor 
compatabilities has a second implication: while there is a 
great deal to be known about the connection between tactical 
input symbols and communicative goals or cognition, it is 
not the tactical generator's job to know it. Its input symbols 
should reflect utterance goals or the processes of deciding 
what to say only to the extent that these are slated for 
explicit expression -- for instance, as the explicit markings 
of this or that speech act. A strategic program, when 
delivering an input structure to the tactical generator, should 
supply in its symbols and their interrelationships all the 
information necessary to make meaning-or-function- 
preserving expressor choices, but it need not supply a full 
trace of the strategic reasoning, and we should not expect to 
7th International Generation Workshop • Kennebunkport, Maine ° June 21-24, 1994 
read the speaker's full motives, word selection process, and 
so on in the tactical input without reading between the lines. 
When the fox tells the crow his voice is beautiful, we will 
see nothing of his ulterior motives in the tactical generation 
input, though presumably they lurk in prior structures 
leading to it. At that level, we see only an assertion, 
indicating some speech goals to be sure, but only such as the 
fox decided would be politic. 
We are not, of course, suggesting that strategic problems be 
ignored -- only that they can and should be cleanly separated 
from tactical ones. It is clear that true semantic and 
pragmatic studies -- studies showing how a given meaning or 
function (set of equivalent expressors) is selected in context 
-- must inform strategic programs. (Beyond this, they are 
desirable for tactical generation efforts as well, since 
semantic/pragmatic analysis can make more plausible a set 
of discourse relation symbols discovered via speakers' 
equivalence judgments. For this purpose, we semantically 
analyze several relations in \[Seligman 1991\] comparing our 
analysis with several competing ones.) But while 
semantic/pragmatic study is indispensable, it is 
methodologically valuable to separate it from synonym- 
based discovery of relations and to treat the discovery as 
primary, as a first step. One can first determine that native 
speakers judge several expressions to have the same 
meaning or function in certain contexts, and then use this 
information as a guide in investigating the nature of the 
equivalence -- the linkage to situations and goals. 
Assuming that the purpose of tactical semantic and 
pragmatic symbols is precisely to capture paraphrases -- to 
symbolize what is semantically and pragmatically invariant 
in a range of surface expressions -- it is natural that a symbol 
set is to be tested by generating purported paraphrases from 
it. Various versions of the output should indeed be judged to 
preserve meaning and pragmatic function. If not, another 
cycle of elieitation must tease apart distinctions or 
commonalities missed the last time round. Such iterative 
semantic investigation is not new, of course: it has been 
termed experimental semantics by \[Hutchins 1971, 1975, 
Leech 1970, 1974, Mel'chuk and Zholkovski 1970\] and 
others. The present modest proposal is to extend this sort of 
investigation to discourse relations and rhetorical types as 
well as to relations in general. And in fact, we will 
characterize discourse relations as meta-relations, not 
different in kind from tile lower-order relations they 
dominate. 
We now examine and compare three specific paraphrase- 
based discovery procedures for discourse relations. We can 
roughly characterize these as semantic meta-relations -- 
second-order relations, relations which relate relations -- 
postponing further discussion until the discovery procedures 
have been introduced. 
THE S-RELATORS OF IVIR EEl" AL 
In a study unfortunately little known, \[Ivir et al 1973\] 
categorize with uncommon completeness numerous "S- 
relators" -- syntactic structures associated with identifiable 
semantic links among the meanings of sentences or clauses. 
Their catalog was compiled as part of a contrastive study of 
English and Serbo-Croatian, and thus strives for cross- 
linguistic universality. 
They began their study by compiling a list of surface clause 
relators from the Brown corpus and standard grammatical 
sources. Their method for discovering discourse relations by 
2 
comparison of such surface expressions is described only as 
follows: 
... we chose sets of two sentences \[in each language\] 
and applied the test of whether a particular item 
\[surface structure\] could link them without 
substantially altering the relationship indicated by 
other members of the same category. What exactly 
constituted a "substantial" alternation, of course, 
presented a complex problem, which we dealt with by 
appealing to the intuition of native speakers \[Ivir et al 
1973:29\]. 
The resulting S-relators, some 62 of them, are listed with 
testing frames taken to be diagnostic and sample relating 
structures. Here are two, omitting the equivalent Serbo- 
Croatian: 
Contrast 
Relator implies that $2 contrasts strongly with S1. 
That material is rather heavy. This material is light. 
In~by (way oj9 contrast 
Manner of causation 
Relator implies that $2 is brought about by means of $1. 
He lost control of the car. He knocked down the power line. 
thus 
and so 
in this way~manner 
thereby 
that is how~the way 
Other relators, intuitively grouped in the study according to 
semantic field, include NON-REAL.CAUSE (as if), 
COMPARATIVE.DEGREE ( so ... that, to such a degree), 
and fifty-odd others. All these relations are listed along with 
a few typical expressors in an appendix to \[Seligman 91\]. 
Later studies were planned to provide fuller expressor 
listings, detailed syntactic analysis of the expressors, and 
stylistic comments concerning them, e.g. "colloquial use 
only". However, it has not been possible to obtain them or 
determine if they were completed. 
\[Seligman 1991\], while accepting the basic premise of the 
procedure just described, attempted to fill it out -- to make it 
more explicit and complete. In particular, Ivir et al's brief 
methodological description left unclear how expressive 
structures can be discovered which are not simple 
conjunctions. Our procedure is designed to enable such 
discovery by eliciting free paraphrase in addition to simple 
syntax-preserving substitutions. 
The following methodology was suggested: 
• Compile a list of surface clause relators. Make it as 
complete as possible. 
From native speakers, elicit plausible examples for 
each expressive structure. For because, the example 
Fred failed the test because he didn't study might be 
elicited. 
Elicit paraphrases of the examples, stipulating that the 
meaning should remain unchanged so far as possible. 
(We take the position that, while true paraphrase may 
7th International Generation Workshop • Kennebunkport, Maine • June 21-24, 1994 
be rare owing to differences in emphasis, associations, 
register, etc., truth value can indeed remain invariant 
among a series of alternate utterances. There is some 
evidence \[Leech and Pepicello 1972\] that speakers can 
observe such distietions. See also \[Leech 1974\] and 
\[Seligman 1979\].) For the present example, the 
following paraphrases might be among those elicited: 
Because Fred didn't study, he failed the test. 
Fred failed the test because of not studying. 
Fred failed the test due to not studying. 
Fred did not study, so he \]'ailed the test. 
Fred did not study. Consequently, he failed the test. 
Fred~ not studying caused his failure on the test. 
Fred's failure on the test was the result of his not 
studying. 
The paraphrases may bring to light expressive 
structures other than clause relators, e.g. because of and 
due to, which relate clauses to noun phrases. Add these 
to the master list, and repeat the above steps for them. 
Paraphrases based on a given structure (like because) 
may employ other structures in the master list (like so, 
consequently, etc.). By crosszcorrelating these 
patterns, sort the expressive structures into proposed 
synonymy groups. 
<CLAUSE> BECAUSE <CLAUSE> 
<CLAUSE> BECAUSE OF <NP> 
<CLAUSE> DUE TO <NP> 
<CLAUSE> SO <CLAUSE> 
<CLAUSE>CONSEQUENTLY <CLAUSE> 
<NP> CAUSE <NP> 
<NP> BE TIIE RESULT OF <NP> 
Assign a name to the relation assumed to underlie each 
such group -- in the present case, CAUSATION is a good 
mnemonic candidate, though RELATION.1 would also 
do -- and the preliminary semantic analysis of discourse 
relations is complete. 
Notice that a given expressive structure can belong to 
several synonymy groups -- can be polysemous, in other 
words. The expression so, for example, can express not 
only CAUSATION (He lost too much blood so he died) but 
also REASON (I'm studying so I won?flunk). 
KNOTT AND DALE'S PROCEDURE 
\[Knott and Dale 1992\] independently proposed a discovery 
procedure for discourse relations which is quite similar to the 
\[Seligman 1991\] proposal just discussed. 
... a classification of phrases according to their 
function in discourse is our central objective .... In 
keeping with the data-driven methodology adopted 
thus far, the classification will be made by means of a 
simple linguistic test, rather than by making 
theoretical claims about the semantics or pragmatics 
of the phrases in the corpus. The test is to do with 
substitutability. Very broadly, if two phrases are inter- 
substitutable in a passage of discourse than they 
should be classified in the same category. \[Knott and 
Dale 1992:18\] 
The study also goes beyond the \[Seligman 1991\] proposal in 
some respects. The most important refinement permits the 
principled discovery of a hierarchical taxonomy of discourse 3 
relations, rather than the flat set of categories given by our 
procedure or the intuitively arranged taxonomy of Ivir et al. 
The key is to note relations of hyponymy as well as 
equivalence among the expressions studied. 
If one phrase can always be substituted for another, but 
not vice versa, then the latter phrase should be 
classified in a category subordinate to that of the 
former phrase. In this way a taxonomy of synonyms 
and hyponyms can be constructed. \[Knott and Dale 
1992:18\] 
This addition is quite valuable. First, it promises to display 
relationships among discourse relations more clearly and 
economically. Representation of these relations in a frame- 
based inheritance hierarchy becomes increasingly attractive, 
for instance. (The advantages of this representational format 
for linguistic objects in general are argued throughout 
\[Seligman 1991\].) But it also points up once again an 
underlying similarity between discourse relations and 
relations in general: discourse relations are relations, and 
subject to the same analytical techniques. Hyponymy 
relations, long noted among lexieal elements, can be 
recognized among discourse relators as well. 
Recognizing such hyponymy relations among discourse 
relators, we realize that one strategic task is to choose a 
degree of specificity for relations at the discourse level as for 
first order relations. The choice between in short or the 
more specific in conclusion, then, is similar to that between 
break and the more specific shatter. At the extreme, certain 
expressors like full stop or semicolon would seem to be so 
general as to express only that some unspecified relation 
obtains. 
On the other hand, another feature of the K&D procedure may 
detract from its flexibility and generality. Like the Ivir et al 
procedure described above, the Knott and Dale substitutions 
preserve syntactic structure. While their procedure does 
permit appropriate variation in the position of a substitute 
expression, it does not accommodate (as our procedure does) 
very free paraphrases in which a relating expression is, for 
example, sometimes a coordinator, sometimes a verb, 
sometimes a preposition, etc., with appropriate syntactic 
changes in the related elements (John is fat because he eats 
too much. John's overeating causes his obesity. John is fat 
because of his overeating.) Thus, whereas our procedure 
starts with conjunctions and spreads toward meaning-related 
expressions of any class, that of Knott and Dale purposely 
begins with and keeps to coordinators, subordinators, 
conjunct adverbs, and certain phrases taking sentential 
complements. (A clear procedure is described for selecting 
these relators at the outset \[Knott and Dale 1992:14-15\].) A 
compromise procedure which allows free paraphrases while 
recognizing hyponyms might provide the best of both 
worlds. 
Based upon this methodology for discovery, we believe with 
Ivir et al and with Knott and Dale that many discourse 
relations can be recognized. (Ivir et al list more than 60; 
K&D group relators into about 45 categories.) Recall that we 
characterized discourse relations as meta-relations. Speakers 
of a language recognize and conventionalize thousands of 
first-order relations; why should they not also recognize 
many relations among relations? Accordingly, in the terms 
of \[Hovy 1990\], we are."profligate" seekers of discourse 
relations. (By contrast, the relatively "parsimonious" Mann 
and Thompson posit some 25-30, while Hobbs employs 
only eight.) We would not be surprised if there were a 
hundred or more, considering that we recognize additional 
7th International Generation Workshop * Kennebunkport, Maine • June 21-24, 1994 
distinctions beyond those seen by Ivir et al. (Specifically, 
we see numerous fine distinctions within the semantic field 
of but, however, etc. which Ivir et al do not observe. They 
list seven relations where we see 15-20 \[Seligman 1986\].) 
We now turn from discourse relations toward a different class 
of linguistic objects, roughly that of speech acts. 
2. RHETORICAL TYPES 
To realistically generate paraphrases of a sample discourse 
below, we will need to recognize semantic/pragmatic objects 
we term rhetorical types. Following are the textual segments 
in question, each accompanied by paraphrases. 
• Tempting as it may be, we shouldn't embrace every 
popular issue that comes along.. 
Although, granted, it is tempting, we shouldn't 
embrace every popular issue that comes along. 
I grant you that it is tempting, but we shouldn't 
embrace every popular issue that comes along. 
True, it is tempting, but we shouldn't embrace every 
popular issue that comes along. 
• Let's be clear: I personally favor the initiative and 
ardently support disarmament negotiations to reduce the 
risk of war. 
Let me make one thing perfectly clear: I personally 
favor the initiative and ardently support disarmament 
negotiations to reduce the risk of war. 
are sometimes called "semantic" in current discussion. We 
avoid this term here as overburdened.) 
Specification of non-rhetorical circumstances alone will not 
suffice to predict the felicity of any utterance. Having 
determined that the speaker's aunt's pen is on the table, we 
cannot predict the appropriateness of her utterance My 
aunt's pen is on the table. Rather, the utterance will be 
appropriate if these non-rhetorical circumstances obtain and 
the speaker believes the audience does not know about them, 
ought to know about them, etc. -- in other words, when all 
the rhetorical circumstances (felicity conditions) for an 
assertion also obtain \[Searle 1969\]. In fact, it is just 
because a great many utterances share these same rhetorical 
circumstances that we find it worthwhile to distinguish a 
subclass of utterances called ASSERTIONS. Other utterance 
subclasses specified in terms of the rhetorical circumstances 
they share are recognizable as speech acts in the tradition of 
\[Austin 1962\] and \[Searle 1969\] -- assertions, questions, 
commands, requests, etc. We will instead speak of 
"rhetorical types", however, because we want to include 
utterance classes not universally considered in the speech act 
literature. In the cases at hand, these are subclasses of 
"classical" speech acts. For example, FORMAL.REQUESTS 
can (obviously enough) be seen as a subclass of REQUESTS. 
Similarly, we view CONCESSIONS as a subclass of 
ASSERTIONS in which the specification is added that the 
speaker grudgingly recognizes the truth of the conceded 
assertion. And we view CLARIFICATIONS as a subclass of 
ASSERTIONS in which the speaker explicitly signals 
apprehension that the assertion is needed to clear up 
misunderstandings which may have been engendered by the 
foregoing. 
1 personally favor the initiative and ardently support 
disarmament negotiations to reduce the risk of war, I 
want that to be quite clear. 
• I urge you to vote against a CCC endorsement of the 
Nuclear Freeze Initiative. 
1 request that you vote against a CCC endorsement of 
the Nuclear Freeze Initiative. 
I ask you to vote against a CCC endorsement of the 
Nuclear Freeze Initiative. 
We establish paraphrase groupings here in order to 
distinguish rhetorical types, just as we did for discourse 
relations -- and just as we might do for semantic objects in 
general. The more reliably we can predict a paraphrase set 
(e.g. true, <clause> or granted, <clause>, etc.), the 
more certain we can be that a distinct underlying pragmatic 
object has been identified. We will refer to the symbol 
yielding the first grouping as CONCESSION. The second and 
third groupings will respectively be generated via the 
symbols CLARIFICATION and FORMAL. REQUEST. 
But just what are rhetorical types? And how are they different 
from standard relations like BELIEVE or discourse relations 
like NON.PREVENTIONS? We now sketch our point of view. 
For any utterance, we can attempt to specify the 
circumstances that make it appropriate. The circumstances, 
however, divide themselves into two important types: those 
relating to the communicative goals of the speech 
participants, which we can term rhetorical circumstances; 
and those relating to everything else, which we can term 
non-rhetorical circumstances. (Non-rhetorical circumstances 
4 
Do discourse relations represent groupings of rhetorical or 
non-rhetorical circumstances? One is apt to opt for 
Rhetorical at first blush: they do help to hold a discourse 
together by relating relations -- hence the name discourse 
relations -- and thus acquire an association with pragmatics. 
But clearly, no communicative goals need be considered to 
recognize a CAUSATION or a SEQUENCE. In our 
conception, then, discourse relations categorize non- 
rhetorical utterance circumstances just as garden-variety 
relations do, and are not to be confused with rhetorical 
types. 
Below, we symbolize the relationship between a rhetorical 
type and an associated relation (of any order) as one of 
dominance. ASSERTION.l, for instance, might dominate 
HITTING.1. Other possible representations are considered in 
\[Seligman 1991\]. 
3. PARAPHRASE-BASED DESCRIPTIONS VS. RST 
Having described an approach to discourse relations and 
rhetorical types (finely subcategorized speech acts), we are 
now in a position to very briefly discuss the relationship 
between these objects and those of certain current discourse 
theories. We concentrate on the Rhetorical Structure Theory 
of \[Mann and Thompson 1983, 1984, 1985\] as especially 
influential, but our comments will apply equally to related 
theories in which the central concern is an account of the 
speaker's goals. Knott and Dale provide a good starting 
point: although they describe their own substitution-based 
discourse relator categories as "pre-theoretical", they have 
noted some discrepancies between these categories and the 
relations of RST. 
It should be noted that the categories of cue phrases 
can often be mapped roughly back onto relations in 
RST's original set. Relations like sequence, contrast, 
7th International Generation Workshop 
circumstance, cause, and result all find natural 
correlates in the taxonomy .... At the same time, there 
are significant differences. These are sometimes to do 
with granularity: for instance, cue phrases marking a 
sequence relation do not form a single group, but are 
themselves taxonomized. Again, the distinction 
between volitional-result and non-volitional-result 
finds no correlate in the taxonomy. On the other hand, 
some RST relations do not appear to have any 
relational phrases associated with them. Evaluation, 
for instance, seems to fall into this class. And 
elaboration, while a prominent relation in many RST 
analyses, seems to have no single phrase to mark it -- 
a particularly surprising finding. \[Knott and Dale 
1992:21\] 
These mismatches come about naturally. RST is primarily 
concerned with the speaker's goals. When an analysis is 
complete, one hopes to have inferred an indication of the 
underlying goals which gave rise to the discourse. Surface 
expressions are viewed primarily as clues to these, albeit 
very imperfect clues in that they are often ambiguous or 
missing. One views speaker goals through surface 
expressions as through a glass darkly. So an RST analysis 
gets what help it can from the surface expressions, but 
depends mainly upon plan inference. Now, given the aim of 
finding deeper rhetorical goals and given the indirectness of 
the link between goals and cue phrases, it should not be 
surprising that when an RST structure is used for generation, 
it is very likely to underspecify or not specify the particular 
language to be used. To the extent that RST-style 
descriptions concentrate on rhetorical goals like evaluation 
or elaboration which may never be explicitly expressed, 
they are more appropriate for strategic than for tactical 
description. 
By contrast, a paraphrase-based structure, when it is used for 
generation, should indeed give a full specification of the 
available linguistic choices. But in compensation, as 
already dramatized with fox and crow, it will not display all 
the planning that went into the decisions about what to 
express. 
A different metaphor may be helpful in conceptualizing the 
difference between a goal-oriented description of an 
utterance and one which is oriented toward explicit linguistic 
expression. When building a house, the contractor expects a 
blueprint specifying its structure in sufficient detail for 
actual construction, though some degrees of freedom usually 
remain. One certainly has many goals in building the house, 
including the meeting of many constraints -- financial ones, 
those of site geography, and so on. But these do not appear 
in the blueprint. The blueprint, rather, is the detailed work 
specification resulting from -- the output of, the trace of -- 
the planning to meet those goals. Of course, one can infer 
much about the goals by examining this work specification, 
but does not expect them to be included within it. One can 
guess a lot about a homeowner's financial constraints by 
looking at ablueprint; but one does not expect to see there 
the notation, "Build this house for under X dollars", or the 
owner's bank balance. Conversely, the builder would not get 
sufficient guidance for actual construction by looking at a 
high-level description of the owner's housing goals. 
It was said that some degrees of freedom usually remain. 
Blueprints can vary in the degrees of specification they 
provide and the degrees of freedom they leave. We can for 
instance imagine a blueprint as made up of several overlays. 
There is the electrical wiring diagram, the piping, etc., in 
addition to the framework. If one overlay were missing, the 
5 
• Kennebunkport, Maine ° June 21-24, 199,4 
builder would be underguided or underconstrained. Likewise, 
an input structure for tactical generation may ideally contain 
several overlays, and be underconstrained for the lack of one 
or more. The network of Figure 4, in particular, does not 
indicate thematic information or salience. Consequently, it 
underconstrains the ordering and thematic structures of the 
output, giving rise to paraphrases which vary freely in these 
respects. (\[Hovy 1993\] urges a many-factored view of 
generation input.) 
Notice finally that the choices of surface expression for 
discourse relations and rhetorical types available to a 
tactical program will often include ambiguous expressors. 
So, as we saw, can express CAUSE, PURPOSE, and other 
relations. So even a relatively full specification from the 
generation viewpoint can lead to language which would be 
ambiguous for an analysis program. Expressors can also be 
unhelpfully general in meaning from an understander's 
viewpoint, with full stop as an extreme example. An 
analysis routine may in such eases need a wide range of 
knowledge to determine which tactical symbol gives a better 
interpretation, or what more specific symbol could have 
been used instead. However, knowledge of possible 
mappings between expressors and semantic/pragmatic 
symbols, and of the full range of conventional discourse 
relations and rhetorical types, should provide some 
constraint. To this extent, knowledge about symbols at the 
tactical level should give at least some help in ultimately 
working back toward the rhetorical goals that gave rise to 
the discourse. 
4. HIERARCHIES OR NETWORKS? 
We have discussed symbols for discourse relations and 
rhetorical types in tactical input structures. We turn now to 
the arrangement of these dements. To give a preliminary 
idea of our motivations for employing graphs rather than 
hierarchies, consider the story of Ted, who drank too much 
and got sick, but gamely went to work anyway. Figure 1 
(top) shows our preferred representation for the underlying 
set of relations. A fuller account of the relations 
CAUSATION and NON.PREVENTIONS appears in \[Seligman 
1991\]. We assume they are sufficiently self-explanatory for 
the present. For comparison, Figure 1 (bottom) displays a 
hierarchical version of the same set of relations. 
We would argue that the hierarchical version distorts the 
relationships in question. It was not the causal relation 
between Ted's drinking and becoming sick that failed to stop 
him from heroically going to work. It was his becoming 
sick which failed to prevent his going ahead. However, if we 
are bound by convention to offer only hierarchical 
representations for this complex of relations, there will be 
no way of avoiding such distortion. It is this convention we 
are arguing against. 
We do not argue that discourse segments can never take on a 
hierarchical form. The point is rather that hierarchies appear 
as special cases in relation networks, and that non- 
hierarchical connections can be formed as well. 
Let us reconsider the same points in the context of a 
somewhat larger and less artificial network. We will consider 
a fragment from a text studied by Mann and Thompson, 
given in full in Section 5. 
Tempting as it may be; we shouldn't embrace every 
popular issue that comes along. When we do so, we use 
precious limited resources where other players with 
superior resources are already doing an adequate job. 
Rather, I think we will be stronger and more effective 
7th International Generation Workshop • Kennebunkport, Maine • June 21-24, 1994 
if we stick to those issues of governmental structure 
and process, broadly defined, that have formed the core 
of our agenda for years. Open government, campaign 
finance reform, and fighting the influence of special 
interests and big money -- these are our kinds of 
issues. 
M&T's analysis of the fragment appears in Figure 2. For 
comparison, our own analysis appears in Figure 3. 
M&T see this entire fragment as instantiating a single 
discourse schema, THESIS/ANTITHESIS. Thus the fragment 
divides itself into something the writer identifies with and 
something he does not identify with. 
In our analysis, by contrast, there is no overarching 
THESIS/ANTITHESIS relation. The various relations in the 
fragment are linked together, to be sure; but it is not because 
they are all under a single roof. Rather, they are linked as 
towns are linked in a road system: any destination can be 
reached from any other, sometimes in more than one way. 
Compare also the EVIDENCE relation of M&T to the 
REASON.FOR.BELIEF relation of our analysis. In both 
analyses, there is a claim to be supported, namely the 
proposition we shouldn't embrace every popular issue that 
comes along. We see only the SHOULDN'T proposition as 
the claim. But M&T treat this proposition as embedded 
within another relation, so that the latter is represented as 
the thing to be proved. We find this indirectness unintuifive, 
an artifact of the dictum that only hierarchical arrangements 
are allowed. (Similarly, we earlier rejected a straw-man 
analysis of the Ted discourse in which the relationship 
between NON.PREVENTION.1 and BECOMING.1 was 
obscured by an intervening layer of structure.) 
With this orientation, we are now prepared to present several 
arguments against a purely hierarchical discourse 
representation. 
The existence proof. Some discourses seem 
coherent because there are interconnections among 
their major parts, even though there would seem to be 
no identifiable overarching schema in which they all 
participate. We offer the Ted mini-discourse, Ted drank 
too much, so he got sick, but he went to work, as an 
example. It seems to be organized more like a three-link 
chain than a tree. If this is granted, we will have found 
at least one example of a coherent but non-hierarchical 
discourse. 
The reductio ad absurdum. We doubt that a single 
overarching schema can be discovered for a very long 
discourse like a novel. If one can be found, we are afraid 
it will be too general to be useful. In particular, it is not 
likely to have expressions predictably associated with 
it, once upon a time notwithstanding. It might be 
countered that discourses are hierarchical only up to a 
certain size. But as soon as one grants the need for some 
non-hierarchical mechanism to relate the largest 
hierarchies, our case is made. 
The handling of reference. Graphs are often 
preferred over trees to represent situations in which a 
single entity performs multiple roles. In the CCC Text, 
for example, the proposition/relation underlying (we) 
embrace every popular issue that comes along plays a 
part in three higher-level relations: it is tempting to .... 
we shouldn't .... and if ... then we will use limited 
resources. Our representation handles this multi- 6 
reference quite naturally, by having three different links 
converging on the same relation. There can be no 
converging in the hierarchical representation. Indeces 
could be used to show the convergence; but this is extra 
machinery we would rather do without. 
Directness of connections as an aid to 
record-keeping. M&T recognize that within any 
span there is a proposition that epitomizes its function 
\[Mann and Thompson 1984:6\]. We think that such a 
most-functional proposition should itself be recognized 
as the direct argument, or terminal, of t_he relation in 
question. The relationships of span-mates to the object 
on the business end of a relation should be represented 
directly, via other specific relations. Such a network of 
direct and specific connections w~ll facilitate record- 
keeping when discourse segments are scrambled during 
realization, especially when elements belonging to 
separate M&T spans intermix. 
Can there be degrees of coherence? Arguing for 
an exclusively hierarchical treatment, M&T argue that, 
if any single relation is removed, the network as a 
whole is split into pieces: it is no longer coherent 
\[Mann and Thompson 1985:12\]. In our treatment, a 
network may or may not be sundered when a single link 
is cut, since there may be alternative routes available 
between subnets. Its coherence will indeed be reduced, 
since one less relation is made explicit, but this is a 
matter of degree. And it should be. A discourse, it seems 
to us, is not usually judged either totally coherent or 
totally incoherent. Rather, it may be more or less 
coherent. 
A network analysis does not preclude the recognition of 
hierarchical part-to-whole relationships within a discourse. 
On the contrary, the structure appears in the net's topology. 
Consider a transportation network, in which roads cluster 
and converge to form obvious hubs, linked by sparse trunk 
lines. In a discourse network, relations likewise define 
bustling hubs and lonely highways. The hubs, or sets of 
nodes linked by many connections, are major parts or 
divisions within the net. Relatively few connections run 
between them. Further, just as there may be sub-hubs within 
a large city -- the Port Authority terminal in New York City, 
for example -- discourse network topology can define parts 
within discourse sections, so that the familiar sort of part- 
within-part hierarchy can be recovered. Further, discourse 
parts or segments, once identified in this way, might 
certainly be associated on a strategic level with rhetorical 
purposes in the manner of \[Grosz and Sidner 1986\]. 
5. EXAMPLES 
For illustration, we now turn to a short attested discourse 
studied by \[Mann and Thompson 1983, 1984, 1985\] and to 
description of the discourse relations and rhetorical types we 
find necessary for tactical generation of it. A sample lattice 
composed of these will be shown, and one alternate version 
of the discourse which could be generated, assuming a 
suitable grammar and procedure, e.g. that of \[Seligman 
1991\]. 
THE CCC TEXT 
We have already seen part of the "CCC Text". A letter-writer 
attempts to convince fellow members of California Common 
Cause to vote down a proposal to support a Nuclear Freeze. 
I don't believe that endorsing the Nuclear Freeze 
Initiative is the right step for California CC. 
7th International Generation Workshop * Kennebunkport, Maine • June 21-24, 1994 
Tempting as it may be, we shouldn't embrace every 
popular issue that comes along. When we do so, we use 
precious limited resources where other players with 
superior resources are already doing an adequate job. 
Rather, I think we will be stronger and more effective 
if we stick to those issues of governmentai structure 
and process, broadly defined, that have formed the core 
of our agenda for years. Open government, campaign 
finance reform, and fighting the influence of special 
interests and big money -- these are our kinds of 
issues. 
Let's be clear: I personally favor the initiative and 
ardently support disarmament negotiations to reduce 
the risk of war. But I don't think endorsing a specific 
nuclear freeze proposal is appropriate for CCC. We 
should limit our involvement in defense and weaponry 
to matters of process, such as exposing the weapons 
industry's influence of the political process. 
Therefore, I urge you to vote against a CCC 
endorsement of the nuclear freeze initiative. 
DISCOURSE RELATIONS AND RHETORICALTYPES 
The discourse relations and rhetorical types we will use to 
represent this text at the tactical level are listed in Appendix 
I along with very brief characterizations, relevant passages 
in the CCC Text, and representative (but incomplete) list of 
expressive patterns, some of which will appear in sample 
generation output below. (\[Seligman 1991\] provides much 
fuller discussion of these and other relations, commenting 
on the differences between our analysis and that of several 
other researchers. Space precludes such treatment here.) 
The relations are grouped for convenience, e.g. as relations 
of the but type, the and type. the/f type, etc. We make no 
claims for the present grouping scheme, but do believe with 
Knott and Dale that relations can be classified according to 
meaning overlaps and differences. (Ivir et al provide an 
intuitive taxonomy close to our present one. It appears as an 
appendix in \[Seligman 1991\].) 
LATTICE FOR TACTICAL GENERATION INPUT AND 
SAMPLE OUTPUT 
The discourse relations and rhetorical types in Appendix I 
can be interconnected to compose the lattice shown in 
Figure 4. 
\[Seligman 1991\] describes a small set of network traversal 
strategies which permits repeated application of a sentence 
grammar to generate paragraphs expressing the whole net. 
An important part of this procedure is the capacity to 
generate bottom-up -- to find well-formed continuations for 
partial structures which are already complete. Certain simple 
facilities for handling anaphora are also specified. Space 
limitations will not permit description of these expression 
procedures here. For concreteness and illustration, however, 
we provide one possible output version, Appendix II. 
Numbers in the network show the ordering in which lattice 
nodes would be visited to produce the discourse variant of 
Appendix II. Numerous other variants (some shown in 
\[Seligman 1991\]) could be produced by varying the traversal 
ordering and selection of expressors. Next to each discourse 
relation (underlined once) or rhetorical type (underlined 
twice) is a parenthesized indication of the pattern used in our 
sample variant. For instance, example (a) indicates the 
choice of the pattern labeled (a) to express the EXAMPLE 
7 
relation. Just below the relation is the chosen expressor 
itself, e.g. "For example". 
Two types of abbreviation in the sample network should be 
pointed out. 
First, in order to concentrate on discourse relations and 
rhetorical types, we use ad hoc semantic tokens as 
terminals. These can in turn receive ad hoc mappings to 
syntactic structures below the clause level. For instance, the 
ad hoe semantic relation IT.IS.TEMPTING might be realized 
only as elause: it is tempting <verb phrase, to 
infinitives. In the sample output text, such fixed text is 
shown in plain typeface. However, discourse-level structures 
generated analytically -- our focus here -- are italicized. 
Secondly, every relation in our network ought to be 
associated with a rhetorical type, since purely non-rhetorical 
circumstances cannot alone justify any utterance. So, just as 
CONCESSION, CLARIFICATION, and FORMAL.REQUEST 
point to their respective relations, all other relations should 
be covered by rhetorical types, here probably ASSERTION 
in each case. For simplicity and readability, however, we 
treat assertions as implied, or unmarked, rhetorical types. 
CONCLUSIONS 
We have argued that a high-level specification of a speaker's 
goals in producing a discourse is not an appropriate input for 
a tactical generator. The latter must assume that some 
process has already decided what to say for whatever reasons, 
and requires a specification of what to say in symbols 
closely tied to potentially usable Surface linguistic 
expressions. These symbols should represent pragmatic or 
semantic ("rhetorical or non-rhetorical") equivalence classes 
of expressions, and should thus be discovered by using as 
primary data speakers' equivalence judgments. We offered a 
discovery procedure based on paraphrase, intended to be 
more explicit and flexible than a procedure suggested by Ivir 
et al. We then compared our proposal with a later, 
independent suggestion due to Knott and Dale, finding that 
the K&D procedure's ability to induce hyponymy as well as 
equivalence relationships was advantageous, but that its 
consideration of only certain syntactic classes as discourse 
relation expressors might be overly restrictive. 
We presented a number of arguments for tactical input 
specifications arranged as lattices rather than strictly as 
hierarchies. 
For illustration, we listed the discourse relations and 
rhetorical types needed to generate an attested paragraph- 
length discourse. We proposed a lattice composed of these as 
tactical generation input, and gave one example of the sort 
of discourse version which could be generated, assuming a 
suitable grammar and set of procedures. 

REFERENCES 
Austin, J. 1962. How to Do Things with Words. Cambridge, 
MA: Harvard University Press, 1962. 
Grosz, B. and C. Sidner. 1986. Attention, Intentions, And 
The Structure Of Discourse. Computational Linguistics, 
1986, 175-203. 
Hovy, E. 1990. "Parsimonious and Profligate Approaches to 
the Question of Discourse Structure Relations." In 
Proceedings of the 5th International Workshop on Language 
Generation, Pittsburgh, PA, June. 
Hovy , E. 1993. "In Defense Of Syntax: Informational, 
Intentional, And Rhetorical Structures In Discourse." In 
Intentionality And Structure In Discourse Relations: 
Proceedings Of A Workshop Sponsored By The Special 
Interest Group On Generation Of The Association For 
Computational Linguistics, Columbus, OH, June. 
Hutchins, W. 1971. The Generation of Syntactic Structures 
from a Semantic Base. London: North-Holland, 1971. 
Hutehins, W. 1975. "Subjects, Themes, and Case 
Grammar."Lingua, 35(2) (1975) 101-133. 
Ivir, V., and D. McMillan, T. Merz. 1973. "S-Relators." 
Yugoslav Serbo-Croatian-English Contrastive Project, 7 
(1973), 22-64. 
Knott, A. and R. Dale. 1992. Using Linguistic Phenomena 
To Motivate A Set Of Rhetorical Relations. Technical 
Report Rp-34. Human Communication Research Centre. 
University Of Edinburgh. 1992. 
Leech, G. 1970. Towards a Semantic Description of English. 
Bloomington: Indiana University Press, 1970. 
Leech, G. 1974. Semantics. Baltimore, MD: Penguin Books, 
Inc., 1974. 
Leech, G. and W. Pepicello. 1972. "Semantic vs. Factual 
Knowledge: an Experimental Approach." In Papers from the 
Symposium on Limiting the Domain of Linguistics. 
Milwaukee: Linguistics Department, University of 
Wisconsin-Milwaukee. 
Mann, W. and S. Thompson. 1983. Relational Propositions 
in Discourse. Information Sciences Institute (ISI) Research 
Report ISI/RR-83-115. November, 1983. 
Mann, W. and S. Thompson. 1984. Discourse Structure for 
Text Generation. Information Sciences Institute (ISI) 
Research Report ISI/RR-84-127. February, 1984. 
Mann, W. and S. Thompson. 1985. Discourse Structure for 
Text Generation. Information Sciences Institute (ISI) 
Research Report ISI/RR-85-155. April, 1985. 
Mel'chuk, I. and A. Zholkovski. 1970. "Toward a 
Functioning Meaning-Text Model of Language." 
Linguistics, 57 (1970), 10-47. 
Searle, J. 1969. Speech Acts. Cambridge: Cambridge 
University Press, 1969. 
Seligman, M. 1979. The Semantic.based Grammar of W.J. 
Hutchins and Some Current lssues in Linguistic Theory. 
Technical Report, Dept. of Linguistics, UC Berkeley. 
Seligman, M. 1986b. But: What and How It Means. 
Technical Report, Dept. of Linguistics, UC Berkeley. 
Seligman, M. 1991. Generating Discourses From Networks 
Using An Inheritance-Based Grammar. Dissertation, Dept. 
of Linguistics, UC Berkeley. 
