DISCOURSE ENTITIES IN JANUS 
Damaris M. Ayuso 
BBN Systems and Technologies Corporation 
10 Moulton Street 
Cambridge, Massachusetts 02138 
dayuso@bbn.com 
Abstract 
This paper addresses issues that arose in apply- 
ing the model for discourse entity (DE) generation in 
B. Webber's work (1978, 1983) to an interactive multi- 
modal interface. Her treatment was extended in 4 
areas: (1)the notion of context dependence of DEs 
was formalized in an intensional logic, (2)the treat- 
ment of DEs for indefinite NPs was modified to use 
skolem functions, (3)the treatment of dependent 
quantifiers was generalized, and (4) DEs originating 
from non-linguistic sources, such as pointing actions, 
were taken into account. The discourse entities are 
used in intra- and extra-sentential pronoun resolution 
in BBN Janus. 
1 Introduction 
Discourse entities (DEs) are descriptions of ob- 
jects, groups of objects, events, etc. from the real 
world or from hypothesized or possible worlds that are 
evoked in a discourse. Any communicative act, be it 
spoken, written, gestured, or system-initiated, can 
give rise to DEs. As a discourse progresses, an ade- 
quate discourse model must represent the relevant 
entities, and the relationships between them (Grosz 
and Sidner, 1986), A speaker may then felicitously 
refer anaphorically to an object (subject to focusing or 
centering constraints (Grosz et al., 1983, Sidner 1981, 
1983, Brennan et al. 1987) ) if there is an existing DE 
representing it, or if a corresponding DE may be 
directly inferred from an existing DE. For example, 
the utterance "Every senior in Milford High School has 
a car" gives rise to at least 3 entities, describable in 
English as "the seniors in Milford High School", 
"Milford High School", and "the set of cars each of 
which is owned by some senior in Milford High 
School". These entities may then be accessed by the 
following next utterances, respectively: 
"They graduate in June." 
"It's a good school." 
"They completely fill the parking lot." 
Webber (1978, 1983) addressed the question of 
determining what discourse entities are introduced by 
a text. She defined rules which produce "initial 
descriptions" (IDs) of new entities stemming from 
noun phrases, given a meaning representation of a 
text. An ID is a logical expression that denotes the 
corresponding object and uses only information from 
the text's meaning representation. The declarative 
nature of Webber's rules and the fact that they relied 
solely on the structure of the meaning representation, 
made her approach well suited for implementation. 
The present work recasts her rules in Janus's in- 
tensional logic framework (described in section 2). 
Two goals guided our approach: (1)that our DE 
representations be semantically clear and correct ac- 
cording to the formal definitions of our language, and 
(2) that these representations be amenable to the 
processing required in an interactive environment 
such as ours, where each reference needs to be fully 
resolved against the current context. 
In the following sections, we first present the 
representational requirements for this approach, and 
introduce our logical language (section 2). 
Then we discuss issues that arose in trying to 
formalize the logical representation of DEs with 
respect to (1) the context dependence of their denota- 
tions, and (2) the indeterminacy of denotation that 
arises with indefinite NPs. For context dependence, 
we use an intensional logic expression indexed by 
time and world indices (discussed in section 3). This 
required us to extend Webber's rules to detect modal 
and other index-binding contexts. In representing 
DEs for indefinites (appearing as existential formulae 
in our meaning representation), we replaced 
Webber's EVOKE predicate with skolem constants for 
the independent case, where it does not contain a 
variable bound by a higher FORALL quantifier 
(section 4), and do not use EVOKE at all in the de- 
pendent case. 
In section 5 we introduce a generalized version of 
the rules for generating DEs for dependent quantifiers 
stemming from indefinite and definite NPs which over- 
comes some difficulties in capturing dependencies be- 
tween discourse entities. 
In our multi-modal interface environment, it is im- 
portant to represent the information on the computer 
screen as part of the discourse context, and allow 
references to screen entities that are not explicitly in- 
troduced via the text input. Section 6 briefly dis- 
cusses some of these issues and shows how pointing 
actions are handled in Janus by generating ap- 
propriate discourse entities that are then used like 
other DEs. 
Finally, section 7 concludes and presents plans 
for future work. 
This is, to our knowledge, the first implementation 
of Webber's DE generation ideas. We designed the 
243 
algorithms and structures necessary to generate dis- 
course entities from our logical representation of the 
meaning of utterances, and from pointing gestures, 
and currently use them in Janus's (Weischedel et al., 
1987, BSN, 1988) pronoun resolution component, 
which applies centering techniques (Grosz et al., 
1983, Sidner 1981, 1983, Brennan et al. 1987) to 
track and constrain references. Janus has been 
demonstrated in the Navy domain for DARPA's Fleet 
Command Center Battle Management Program 
(FCCBMP), and in the Army domain for the Air Land 
Battle Management Program (ALBM). 
2 Meaninq Representation for DE 
Generation 
Webber found that appropriate discourse entities 
could be generated from the meaning representation 
of a sentence by applying rules to the representation 
that are strictly structural in nature, as long as the 
representation reflects certain crucial aspects of the 
sentence. This has the attractive feature that any 
syntactic formalism may be used if an appropriate 
semantic representation is produced. Some of the 
requirements (described in (Webber 1978, 1983)) on 
the representation are: (1) it must distinguish be- 
tween definite and indefinite NPs and between sin- 
gular and plural NPs, (2)it must specify quantifier 
scope, (3) it must distinguish between distributive and 
collective readings, (4)it must have resolved elided 
verb phrases, and (5) it must reflect the modifier struc- 
ture of the NPs (e.g., via restricted quantification). An 
important implied constraint is that the representation 
must show one recognizable construct (a quantifier, 
for example) per DE-invoking noun phrase. These 
constructs are what trigger the DE generation rules. 
Insofar as a semantic representation reflects all of 
the above in its structure, structural rules will suffice 
for generating appropriate DEs, but otherwise infor- 
mation from syntax or other sources may be neces- 
sary. There is a trade-off between using a level of 
representation that shows the required distinctions, 
and the need to stay relatively close to the English 
structure in order to only generate DEs that are jus- 
tiffed by the text. For example, in Janus, in addition to 
quantiflers from NPs, the semantic representation has 
quantiflers for verbs (events), and possibly extra 
quantifiers introduced in representing deeper meaning 
or by the collective/distributive processing. Therefore, 
we check the syntactic source of the quantifiers to 
ensure that we only generate entities for quantifiers 
that arose from NPs (using the bound variable as an 
index into the parse tree). 
Other than the caveat just discussed, the Janus 
meaning representation language WML (for World 
Model Language) (Hinrichs et al., 1987) meets all the 
other constraints for DE generation. WML is a higher- 
order intensional language that is based on a syn- 
thesis between the kind of language used in PHLIQA 
(Scha, 1976) and Montague's Intensional Logic 
244 
(Montague, 1973). A newer version of WML (Stallard, 
1988) is used in the 8BN Spoken Language System 
(Boisen et al., 1989). The intensionality of WML 
makes it more powerful than the sample language 
Webber used in developing her structural rules. 
The scoping expressions in WML have a sort field 
(which restricts the range of the variable) and have 
the form: 
(1= x s (P x)) 
where B is a quantifier such as FORALL or EXISTS, a 
term-forming operator like IOTA or SET, or the 
lambda abstraction operator LAMBDA. S is the sort, 
a set-denoting expression of arbitrary complexity 
specifying the range of x, and (P x) is a predication in 
terms of x. The formal semantics of WML assigns a 
type to each well-formed expression which is a func- 
tion of the types of its parts. If expression E has type 
T, the denotation of E, given a model M and a time t 
and world w, is a member of the set which is T's 
domain. One use of types in our system is for enforc- 
ing selectional restrictions. The formation rules of 
WML, its type system, and its recursive denotation 
definition provide a formal syntax and semantics for 
WML. 
3 Context Dependence of Discourse 
Entities 
A formal semantics was assumed though not 
given for the sample logical language used by Web- 
bar. The initial descriptions (IDs) of DEs produced by 
her rules were stated in this language too, and thus 
are meant to denote the object the DE represents. 
For example, the rule which applies to the represen- 
tation for independent definite NPs assigns to the 
resulting DE an ID which is the representation itself: 
(t x S (P x)) => ID: (t x S (P x)) 
where t is Russell's iota operator. Thus, the ID for 
"the cat" in "1 saw the cat" is (t x cats T). (Since the 
body of the t in this example has no additional 
predication on x, it is merely T, for TRUE.) However, 
because IDs are solely drawn from the meaning 
representation of the isolated text, they may not suf- 
fice to denote a unique object. Connection to prior 
discourse knowledge or information from further dis- 
course may be necessary to establish a unique 
referent, or determining the referent may not even be 
necessary. For example, the ID for "the cat" would 
need to be evaluated in a context where there is only 
one salient cat in orddr to obtain a denotation. 
Our system's representation of a DE is a structure 
containing several fields. The "logical-form" field con- 
tains a WML expression which denotes the object the 
DE'describes (this corresponds roughly to Webber's 
ID). Given that WML is intensional, we are able to 
explicitly represent context dependence by having the 
logical form include an intensional core, plus tense, 
time, and world information (which includes discourse 
context) that grounds the intension so that it may be 
evaluated. For example, the logical form for the DE 
corresponding to "the cat" in our system is 
( (Z~'~'ZNSION (IOTA x eat- T) ) 
time world) 
where time, if unfilled, defaults to the present, and 
world defaults to the real world and current discourse 
state. The semantics of our IOTA operator makes it 
denotationless if there is not exactly one salient object 
that fits the description in the context, else its denota- 
tion is that unique object. In our interactive system 
each reference needs to be fully resolved to be used 
successfully. If unknown information is necessary to 
obtain a unique denotation for a IOTA term, a simple 
clarification dialogue should ensue. (Clarification is 
not implemented yet, currently the set of all values 
fitting the IOTA is used.) 
An example using the time index is the noun 
phrase "the ships that were combat ready on 
12/1/88", which would generate a DE with logical 
form: 
( ( INTENS ION 
(PAST ( INTENSION 
(IOTA x (SETS ,,hips) 
(COMBAT-READY x) ) ) ) ) 
12/1/88 world) 
Representing this time index in the logical form is cru- 
cial, since a later reference to it, made in a different 
time context must still denote the original object. For 
example, "Are they deployed?" must have "they" 
refer to the ships that were combat ready on 12/1/88, 
not at the time of the latter utterance. 
In order to derive the proper time and world con- 
text for the discourse entities, we added structural 
rules that recognize intensional and index-binding 
logical contexts. Our DE generation algorithm uses 
these rules to gather the necessary information as it 
recurses into the logical representation (applying rules 
as it goes) so that when a regular rule fires on a 
language construct, the appropriate outer-scoping 
time/world bindings will get used for the generated 
DEs. 
It should be noted that, as the discussion above 
suggests, a definite NP always gives rise to a new 
discourse entity in our system. If it is determined to 
be anaphoric, then a pointer to the DE it co-refers with 
(when found) will be added to its "refers-to" field, in- 
dicating they both denote the same object. 
4 DEs for Independent Indefinite NPs 
In Webber's work, the initial description (ID) for a 
DE stemming from an independent existential (i.e., 
with no dependencies on an outer FORALL 
quantifier), contained an EVOKE predicate. "1 saw a 
cat": 
(EXISTS x cat8 (maw I x)) 
would generate a DE with ID: 
(t x Gat8 
(& (saw I x) (EVOI~ Sent x))) 
"The cat I saw that was evoked by sentence Sent", 
where Sent is the parsed clause for '1 saw a cat". 
The purpose of EVOKE was to make clear that al- 
though more than one cat may have been seen, the 
"a" picks out one in particular (which one we do not 
know except that it is the one mentioned in the 
utterance), and this is the cat which makes the 
EVOKE true. Any subsequent reference then picks 
out the same cat because it will access this DE. The 
semantics of the EVOKE predicate and the type of the 
S argument (which is syntactic in nature) were un- 
clear, so we looked for a different formulation with 
better understood semantics. 
Predicate logic already provides us with a 
mechanism for selecting arbitrary individuals from the 
domain via skolem functions (used as a mechanism 
for removing existentials from a formula while preserv- 
ing satisfiability). Skolem functions have been used in 
computational linguistics to indicate quantifier scope, 
for example (VanLehn, 1978). Following a suggestion 
by R. Scha, we use skolem functions in the logical 
form of the DE for the "indefinite individuals" intro- 
duced by independent existentials (Scha et al., 1987). 
For clarity and consistency with the rest of the lan- 
guage, we use a sortedskolem form, where the range 
of the function is specified. Since we use this for 
representing existentials that are independent, the 
function has no arguments and is thus equivalent to a 
sorted constant whose denotation is undetermined 
when introduced. (In this sense it is consistent with 
Karttunen's (1976) and Kamp's (1984) view of the 
indefinite's role as a referential constant, but unlike 
Kamp, here the sentence's meaning representation is 
separate from the representation of the evoked 
entity.) 
Thus we introduced a new operator to WML 
named SKOLEM, for expressions of the form 
(SKOLEM n <sort>), where n is an integer that gets 
incremented for each new skolem created, as a way 
of naming the skolem function. For the example 
above, the core logical form (stripping the outer inten- 
sion and indices) for the DE of "a cat" would be: 
(SKOL~M I (SET x oats (saw I x))) 
denoting a particular cat from the set of aJl the cats I 
saw. The type of a SKOLEM expression is well- 
defined and is given by the following type rule: 
TYPEO¥ (SKOZJCN Ib"~G~S (SETS a)) 
= a 
where INTEGERS is the type for integers, and (SETS 
a) is the type of sets whose members have type a. 
This type rule says that when the first argument of 
SKOLEM is of type INTEGER, and the second is a set 
with elements of type a, then the type of the SKOLEM 
expression is a. Therefore, the type of the above 
example is cats. The explicit connection to the 
originating sentence which the EVOKE predicate 
provided is found in our scheme outside of the logical 
245 
representation by having a pointer in the DE's struc- 
ture to the parse tree NP constituent, and to the struc- 
ture representing the communicative act performed by 
the utterance (in the fields "corresponding-constituent" 
and "originating-communicative-act", respectively). 
These connections are used by the pronoun resolu- 
t/on algorithms which make use of syntactic infor- 
mation. 
Does the denotation of a skolem constant ever get 
determined? In narrative, and even in conversation, 
identifying the individual referred to by the indefinite 
NP frequently doesn't occur. However, in our inter- 
active system, each reference must be fully resolved. 
When the evaluation component of Janus determines 
a successful value to use for the existential in the 
text's logical form, the appropriate function denotation 
for SKOLEM n gets defined, and the "extension" field 
is set for the discourse entity. 
Note that many interesting issues come up in the 
treatment of reference to these indefinite entities in a 
real system. For example, cooperative responses by 
the system introduce new entities that must be taken 
into account. If the user asks "Is there a carrier within 
50 miles of Hawaii?", a cooperative "There are two: 
Constellation and Kennedy" (as opposed to just 
"Yes") must add those two carriers as entities, which 
now overshadow the singular skolem entity for "a car- 
der within 50 miles of Hawaii". On the other hand, a 
"No" answer should block any further reference to the 
carrier skolem, since its denotation is null, while still 
allowing a reference to a class entity derived from it, 
as in "Is there one near San Diego?" where one refers 
to the class carriers. 
The treatment presented works for straightforward 
cases of independent indefinites. Trickier cases like 
donkey sentences (Kamp, 1984, Webber, 1981) and 
interactions with negation have not yet been ad- 
dressed. 
5 Dependent NPs 
5.1 Dependent Indefinite NPs 
Our work uncovered a need for modifications in 
Webber's structural rules for quantifiers from indefinite 
and definite NPs which have dependencies on vari- 
ables bound directly or indirectly by an outer FORALL 
quantifier. In this section we address the case of 
dependent existentials arising from indefinite NPs. 
We first argue that the predicate EVOKE is not 
needed in this context. Then we point out the need 
for generalizing the rule to take into account not just 
FORALL, but all scoping operators that intervene be- 
tween the outer FORALL and the inner EXISTS. 
Finally, we show that the dependencies between dis- 
course entities must be explicitly maintained in the 
logical forms of newly created DEs that depend on 
them. 
Webber's rules are designed to apply from the 
outermost quantifier in; each time a rule is applied the 
remaining logical form is modified to be in terms of the 
just created DE. For example, "Every boy saw a girl 
he knows" has logical form (for the bound pronoun 
reading): 
(FOR~LL x boys 
(EXISTS y (SET y' girls 
(knows x y' ) ) 
(SaW x y) ) ) 
The first step is to apply the rule for an independent 
universal quantifier: 
R0: (FORALL x S (P x)) => de: S 
This application yields the entity for "the set of all 
boys" 
DE I : boys 
and we rewrite the logical form to be: 
(FORALL x DE 1 
(EXISTS y (SET y' girls 
(knows x y')) 
(saw x y) ) ) 
The steps shown so far are consistent with both 
Webber's and our approach. Now we want to apply 
the general rule for existentials within the body of a 
distributive, in order to generate an entity for the 
relevant set of girls. Webber uses Rule 3 in (Webber, 
1983) (here corrected to position the existential's sort 
S inside the scope of the outer quantifiers in the 
generated DE): 
R3: (¥O~,~.lr.,L YI"''Yk 
(EXISTS x s (P x))) => 
de: (SET x things 
(EXISTS YI" • "Yk 
(a (msmbQr x S) (P x) 
(EVOKE Ssnt x) ) ) ) 
where FORALL Yl""Yk is shorthand for FORALL Yl 
de 1 (...(FORALL Yk dek, analogously for EXISTS, and 
S or P depends directly or indirectly on Yl ""Yk' 
Now the first DE we want to generate with this rule 
is for "the set of girls, each of which is known by some 
boy in DE 1, and was seen by him". Does each girl in 
the set also have to satisfy an EVOKE predicate? It 
seems that any future reference back to the set 
formed by the existential seeks to obtain a/I items 
fitting the description, not some subset constrained by 
EVOKE. For example, if the example above is fol- 
lowed by "the girls tried to hide", taking "the girls" 
anaphorically, one wants a/I the girls seen by some 
boy in DE 1 that knows them, no less. Our core logical 
representation for the set of girls is thus: 
DEE: (SET y girls 
(EXISTS x DE I 
(a (knows x y) (saw x y)))) 
So the modified rule used in producing DE 2 is: 
246 
R3': (¥ORALL y~...yk 
(EXISTS x S (P x))) => 
de: (SET x S t 
(EXISTS YI"" "Yk 
(a (.--~.r x s) (\]~ x)))) 
where EVOKE has been removed, and the DE's sort 
field is S t for the "root type" of S, which is the type of 
the members of S, in order to appropriately constrain 
the DE's sort (instead of leaving it as the uncon- 
strained "things"). 
A second change that needs to be made is to 
generalize the left hand side of the rule so that the 
scoping expressions outscoping the inner EXISTS in 
the pattern also be allowed to include other scoping 
operators, such as EXISTS and IOTA. As long as the 
outermost quantifier is a FORALL, any other depend- 
ent scoping expression within it will generate a set- 
denoting DE and will behave as a distributive environ- 
ment as far as any more deeply embedded expres- 
sions are concerned. In other words, the distribu- 
tiveness chains along the dependent quantifiers. To 
see this, consider the more embedded example 
"Every boy gave a girl he knew a peach she wanted", 
where there is an intervening existential between the 
outer FORALL and innermost EXISTS. The core logi- 
cal form for this sentence is: 
(FORALL x boye 
(EXISTS y (SET y' girls 
(knowe x ¥' ) ) 
(EXISTS z (SET z' ~aohea 
(wan*:a y z' ) ) 
(gave z y z)))) 
DE 1 would be as above. Using rule R3' DF_. 2 be- 
comes: 
DE 2 : 
(SET y girle 
(EXISTS x DE I 
(a (knowe x y) 
(EXISTS z (SET z' peaches 
(wants Y =') ) 
(gave x y =))))) 
"The set of girls, each of which is known by some boy 
in DE 1, and got a peach she wanted from that boy." 
Now the peach quantifier should generate a set DE in 
terms of DE 1 and DE 2. Applying R3' gives us: 
DE3: (SET z peachee 
(EXISTS x DE I 
(EXISTS y DE 2 
(a (wanta y z) 
(gave x y z))))) 
"The set of peaches z such that there is a girl in DE 2 
(who is known by some boy in DE I, and who got 
some peach she wan.tpd from the boy), who wants z, 
and who got it from some boy in DE 1''. 
Now a third and final problem becomes apparent: 
for the general case of arbitrary embedding of de- 
pendent quantifiers we generate a DE (e.g., DF_,3) de- 
pendent on other DEs from the outer quantifiers, but 
the dependencies between those DEs (e.g., DE 1 and 
DE2) are not maintained. This is counter-intuitive, and 
also leads to an under-specified set DE. In the 
peaches example above, envision the situation where 
a boy b I gave out two peaches Pl and P2 : one to a 
girl gl he knew, and one to a girl g2 he didn't know, 
who also got a peach P3 from another boy b 2 who did 
know her. These are the facts of interest in this 
scenario: 
I. (& (gava b I gl p1) (know b I gl) 
(want= gl Pl)) 
2. (& (gave blg2P2) 
(NOT (know bl gE) ) 
(wanta gEPE) ) 
3. (& (gave bEgEp 3) (know bEgE) 
(wants g2 P3 ) ) 
Since b 1 and b 2 are in DE 1 (due to facts 1 and 3), and 
g2 is in DE 2 (due to fact 3), then P2 is in DE 3 (due to 
fact 2 and according to the DF_. 3 logical form above). 
But P2 should notbe in DE 3, since P2 was NOT given 
to a girl by a boy she knew. The set of peaches 
obtained for DE 3 is too large. The problem would not 
arise if in the DE 3 logical form, the variables ranging 
over DF-- 2 were appropriately connected to DE 1 using 
the dependent restriction present in the original for- 
mula (knows xy). A correct DE 3 is: 
DE 3 : 
(SET z ~:Hmache,= 
(EXISTS x DE z 
(EXISTS y (SET y' DE 2 
(knows x y' ) ) 
(& (want= y =) 
(gave x y z))))) 
To be able to do this, the rule-application algorithm 
must be modified to include the restriction information 
(for dependent restrictions) when the formula gets 
rewritten in terms of a newly created DE. Therefore 
the final generalized rule, which includes other scop- 
ing operators and works on properly connected DEs is 
as follows: 
R3'' : (¥ORALL v I S I 
(Q2 v2 S2 "'" Q. v S= 
(EXISTS x S (P x)))) => 
de: (SET x S t 
(EXISTS v I S I ...v= S 
(~ (mem~r x S) (~ x)))) 
where S or P depend directly or indirectly on v 1...v n, 
Qi may be FORALL, EXISTS, or IOTA, and the scop- 
ing operators outside the inner EXISTS have already 
been processed by any appropriate rules that have 
replaced their original sorts by the Sis, which are in 
terms of generated DEs and explicitly show any DE 
dependencies. The right hand side is as before, with 
existentials picking out elements from each outer 
quantifier. 
247 
act. Since "them" and *it" have different number re- 
quirements, there is no ambiguity and the anaphor 
resolution module resolves "them" to the DE cor- 
responding to "the C1 carriers in the Indian Ocean" 
and "it" to the DE for Kennedy. We are currently 
working on having system-initiated actions also 
generate entities. 
7 Conclusions and Further Work 
Webber's general approach to discourse entity 
generation from a logical representation proved very 
useful in our efforts. We were able to recast her basic 
ideas in our logical framework, and currently use the 
generated DEs extensively. 
The fact that the generation of DEs is done via 
structural rules operating on a semantic represen- 
tation provided a degree of modularity that allowed 
our pronoun resolution component to work 
automatically when we combined a new syntactic 
component with our semantic and discourse com- 
ponent (replacing an ATN by a unification grammar, in 
an independently motivated experiment). We are cur- 
rently starting to port the DE generation component to 
the BBN Spoken Language System (Boisen et al., 
1989), and plan to integrate it with the intra-sentential 
mechanisms in (Ingria and Stallard, 1989). The fact 
that entity representations are mostly semantic in na- 
ture, not syntactic, also facilitated the addition and use 
of non-linguistic entities in a uniform way. 
There are several areas that we would like to 
study to extend our current treatment. We want to 
address the interactions between centering 
phenomena and non-linguistic events that affect dis- 
course focus, such as changing contexts via a menu 
selection in an expert system. 
Our paraphrasing component (Meteer and 
Shaked, 1988) already uses the discourse entities to 
a limited extent. One area of future work is to have 
the language generator make more extensive use of 
them, so it can smoothly refer to focused objects. 
Finally, although quantified expressions are al- 
ready generated in Janus for events implicit in many 
verbs, they are not being used for DEs. We would 
like to address the problem of event reference and its 
interaction with temporal information, using ideas 
such as those in (Webber, 1988) and in the special 
issue of ComputationaJ Linguistics on tense and 
aspect (Vol. 14, Number 2 June 1988). 
8 Acknowledgments 
The work presented here was supported under 
DARPA contract #N00014-85-C-0016. The views and 
conclusions contained in this document are those of 
the author and should not be interpreted as neces- 
sarily representing the official policies, either ex- 
pressed or implied, of the Defense Advanced 
Research Projects Agency or of the United States 
Government. The author would like to thank Dave 
Stallard for invaluable discussions during the writing 
of this paper. Thanks also to Remko Scha, Lance 
Ramshaw, Ralph Weischedel, and Candy Sidner. 
References 
BBN Systems and Technologies Corp. (1988). A 
Guide to IRUS-II Application Development in 
the FCCBMP (BBN Report 6859). Cambridge, 
MA: Bolt Beranek and Newman Inc. 
Boisen, S., Chow Y., Haas, A, Ingria, R., Roucos, S., 
Scha, R., Stallard, D., and Vilain, M. (1989). 
Integration of Speech and Natural Language: 
Final Report (BBN Report 6991 ). BBN Systems 
and Technologies Corp. 
Brennan, Susan E., Friedman, Marilyn W., and Pol- 
lard, Carl J. (1987). A Centering Approach to 
Pronouns. Proceedings of the 25th Annual 
Meeting of the ACL. ACL. 
Grosz, Barbara J., and Sidner, Candace L. (1986). 
Attention, Intentions, and the Structure of Dis- 
course. Computational Linguistics, 12(3), 
175-204. 
Grosz, Barbara J., Joshi, Aravind K., Weinstein, Scott. 
(1983). Providing a Unified Account of Definite 
Noun Phrases in Discourse, Proceedings of 
the 21st Annual Meeting of the ACL. 
Cambridge, MA: ACL. 
Hinrichs, E.W., Ayuso, D.M., and Scha, R. (1987). 
The Syntax and Semantics of the JANUS 
Semantic Interpretation Language. In 
Research and Development in Natural Lan- 
guage Understanding as Part of the Strategic 
Computing Program, Annual Technical Report 
December 1985 . December 1986. BBN 
Laboratories, Report No. 6522. 
Ingria, Robert J.P., and Stallard, David. (1989). A 
Computational Mechanism for Pronominal Ref- 
erence. Proceedings of the 27th Annual Meet- 
ing of the ACL. ACL. 
Kamp, Hans. (1984). A Theory of Truth and Seman- 
tic Representation. In J. Groenendijk. T.M.V. 
Janssen, and M. Stokhof (Eds.), Truth, Inter- 
pretation and Information, Selected Papers 
from the Third Amsterdam Colloquium. 
Dordrecht: Foris Publications. 
Karttunen, Laud. (1976). Discourse Referents. In 
J. D. McCawley (Ed.), Syntax and Semantics, 
Volume 7. New York: Academic Press. 
Meteer, Marie and Shaked. Varda. (1988). Strategies 
for Effective Paraphrasing. Proceedings of 
COLING-88, Budapest, Hungary, August 22-27. 
COLING. 
248 
5.2 Dependent Definite NPs 
Some of the problems described in the previous 
section also arise for the rule to handle dependent 
definite NPs. Definite NPs are treated as IOTA terms 
in WML. (Webber's logical language in (Webber, 
1978) used a similar t. The treatment was later 
changed (Webber, 1983) to use the definite existential 
quantifier "Existsl', but this difference is not relevant 
for the following.) Replacing IOTA for t in Webber's 
(1978) rule 5: 
R5: (FOt~,,L Y~.'''Yk 
(P (IOTA x S (~ x)))) => 
de: (SET z things 
(EXISTS YI"" "Yk 
(m z (IOTA x S (R =))))) 
where Yl'"Yk are universal quantifiers over DEs as in 
R3 above, and S or R depend directly or indirectly on 
Yl"'Yk" 
The second and third extensions discussed in the 
previous section are needed here too: generalizing 
the quantifiers that outscope the inner existential, and 
keeping the dependencies among the DEs explicit to 
avoid under-specified sets. An example of an under- 
specified set arises when the dependent IOTA 
depends jointly on more than one outer variable; for 
example, in "Every boy gave a girl he knew the peach 
they selected", each peach depends on the selection 
by a boy and a girl together. Take a scenario 
analogous to that in the previous section, with the 
facts now as follows (replacing "selected" for "wants*): 
1. (& (gave by gl P~) (know b r gl) 
(8ele,=ted (SETOF bl gl) pr ) ) 
2. (& (gave b t g2P2) 
(NOT (know b I g2) ) 
(=elected (SETO¥ b 1 g2) P2) ) " 
3. (& (gave b292P3) (know b292) 
(=ele¢ted (SETOF b292) P3)) 
By an analogous argument as before, using R5, the 
set of peaches will incorrectly contain P2' given by a 
boy to a girl who selected it with him, but whom he did 
not know. The modified rule is analogous to R3" in 
the previous section: 
RS' : (FORALL v I S I 
(Q= v z s= ... O~ v= s= 
(p (IOTA x s (R x))))) => 
de: (SET z S t 
(EXISTS v I S I ...v S 
(= z (IOTA x S (R x)}))) 
Note that this problem of under-specified sets 
does not arise when the dependency inside the IOTA 
is on one variable, because the definite "the" forces a 
one-to-one mapping from the possible assignments of 
the single outer variable represented in the IOTA to 
the IOTA denotations. If we use the example, "Every 
boy gave a girl he knew the peach she wanted", with 
logical form: 
(FORALL x boys 
(EXISTS y (SET y' gi=is 
(know= x y' ) ) 
(gave x y (IOTA z pea=hem 
(want,, y =) ) ) ) ) 
there is such a mapping between the set of girls in the 
appropriate DE 2 (those who got the peach they 
wanted from a boy they knew) and the peaches in 
DE 3 obtained via R5' (the peaches that some girl in 
DE 2 wanted). Each gid wants exactly one peach, so 
facts 2 and 3, where the same girl receives two dif- 
ferent peaches, cannot occur. So the definite ensures 
that no scenario can be constructed containing extra 
items, as long as there is only one outer variable in 
the inner iota. However in the joint dependency ex- 
ample above using "selected", the one-to-one map- 
ping is between boy-girl pairs and peaches, so the 
relationship between the boys and the girls becomes 
an integral part of determining the correct DE 3. 
6 Non-Linguistic Discourse Entities 
In a dialogue between persons, references can be 
made not only to linguistically-introduced objects, but 
also to objects (or events, etc.) that become salient in 
the environment through some non-linguistic means. 
For example, a loud noise may prompt a question 
"What was that ?", or one may look at or point to an 
object and refer to it, "What's wrong with it ?". It 
seems an attention-drawing event normally precedes 
such a reference. 
In the Janus human-computer environment, non- 
linguistic attention-drawing mechanisms that we have 
identified so far include pointing actions by the user, 
and highlighting (by the system) of changes on the 
screen as a response to a request (or for other 
reasons). The appearance of answers to questions 
also draws the user's attention. We incorporated 
these into generalized notion of a "communicative 
act" which may be linguistic in nature (English input or 
generated English output), a pointing gesture by the 
user, or some other system-initiated action. Any com- 
municative act may give rise to DEs and affect the 
focused entities in the discourse. 
We have implemented procedures to handle 
pointing actions by generating discourse entities 
which are then used in the pronoun resolution com- 
ponent uniformly with the others. For example, after 
the request *Show me the C1 carriers in the Indian 
Ocean" the system will display icons on the color 
monitor representing the carriers. The user can then 
say "Which of them are within 200 miles of it? <point 
with mouse to Kennedy>*. Before the sentence gets 
processed, a discourse entity with the logical form 
(IOTA x carriers (nameof x "Kennedy")) • will be 
created and added to the list of entities currently in 
focus (the "forward looking centers* of the last linguis- 
tic act); the DE's "originating-communicative-act" field 
will point to a newly created "pointing" communicative 
249 
Montague, Richard. (1973). The Proper Treatment of 
Quantification in Ordinary English. In 
J. Hintikka, J. Moravcsik and P. Suppes (Eds.), 
Approaches to Natural Language. Dordrecht: 
Reidel. 
Scha, Remko J.H. (1976). Semantic Types in 
PHLIQAI. Coling 76 Preprints. Ottawa, 
Canada. 
Scha, Remko J.H., Bruce, Bertram C., and Polanyi, 
Livia. (1987). Discourse Understanding. In 
Encyclopedia of Artificial Intelligence. John 
Wiley & Sons, Inc. 
Sidner, Candace L. (1981). Focusing for the Inter- 
pretation of Pronouns. American Journal of 
Computational Linguistics, 7(4), 217-231. 
Sidner, Candace L. (1983). Focusing in the Com- 
prehension of Definite Anaphora. In M. Brady 
and R. C. Berwick (Eds.), Computational 
Models of Discourse. Cambridge, MA: MIT 
Press. 
Stallard, David G. (1988). A Manual for the Logical 
Language of the BBN Spoken Language Sys- 
tem. Unpublished. 
Kurt VanLehn. (1978). Determining the Scope of 
English Quantifiers (Tech. Rep. 483). MIT Ar- 
tificial Intelligence Laboratory. 
Webber, Bonnie L. (1978). A Formal Approach to 
Discourse Anaphora (BBN Report 3761). 
Cambridge, MA: Bolt Beranek and Newman. 
Webber, Bonnie L. (1981). Discourse Model Syn- 
thesis: Preliminaries to Reference. In Joshi, 
Webber, and Sag (Eds.), Elements of Dis. 
course Understanding. Cambridge University 
Press. 
Webber, Bonnie L. (1983). So What Can We Talk 
About Now? In Brady and Berwick (Eds.), 
Computational Models of Discourse. MIT 
Press. 
Webber, Bonnie L. (1988). Discourse Deixis: Refer- 
ence to Discourse Segments. Proceedings of 
the 26th Annual Meeting of the ACL. ACL. 
Weischedel, R., Ayuso, D., Haas, A., Hinrichs, E., 
Scha, R., Shaked, V., and Stallard, D. (1987). 
Research and Development in Natural Lan- 
guage Understanding as Part of the Strategic 
Computing Program, Annual Technical Report 
December 1985- December 1986 (BBN Report 
6522). Cambridge, MA: Bolt Beranek and 
Newman. 
250 
