A COMPUTATIONAL MECHANISM FOR PRONOMINAL REFERENCE 
Robed J. P. Ingria 
David Stallard 
BBN Systems and Technologies, Incorporated 
10 Mouiton Street 
Mailstop 009 
Cambridge, MA 02238 
ABSTRACT the syntactically impossible antecedents. This latter 
This paper describes an implemented mechanism 
for handling bound anaphora, disjoint reference, and 
pronominal reference. The algorithm maps over 
every node in a parse tree in a left-to-right, depth first 
manner. Forward and backwards coreference, and 
disjoint reference are assigned during this tree walk. 
A semantic interpretation procedure is used to deal 
with multiple antecedents. 
1. INTRODUCTION 
This paper describes an implemented mechanism 
for assigning antecedents to bound anaphors and per- 
sonal pronouns, and for establishing disjoint reference 
between Noun Phrases. This mechanism is part of 
the BBN Spoken Language System (Boissn, et al. 
(1989)). The algorithm used is inspired by the index- 
ing scheme of Chomsky (1960), augmented by tables 
analogous to the "Table of Coreference" of Jack- 
endoff (1972). This mechanism handles only intra- 
ssntentJal phenomena and only selects the syntac- 
tically and semantically possible antecedents. Ul- 
timately, it is meant to be used in conjunction with an 
extra-sentential reference mechanism like that 
described in Ayuso (1989) to include antecedents 
from other utterances and to utilize discourse factors 
in its final selection of an antecedent. 
In Section 2 the empirical and theoretical back- 
ground to this treatment is sketched out. In Section 3, 
the actual algorithm used is described in detail. In 
Section 4, the associated semantic interpretation 
mechanism is presented. In Section 5, we compare 
the algorithm with related work. Finally, in Section 6, 
remaining theoretical and implementational issues are 
discussed. 
2. THEORETICAL BACKGROUND 
While most computational systems are interested 
in the potential antecedents of pronouns, work in 
generative grammar by Lasnik (1976) and Reinhart 
(1976) has led to the conclusion that sentential syntax 
is responsible for assigning possible antecedents to 
bound anaphors (reflexives, such as "himself", 
"herself", "themselves", etc., and the reciprocals 
"each other" and "one another") but not to personal 
pronouns ("he", "she", "they", etc). In the case of 
personal pronouns, sentential syntax only determines 
procedure is called disjoint reference, since the im- 
possible antecedents can not even overlap in refer- 
ence with the pronoun; compare the cases in sen- 
tences (1) and (2), where the underlined items are 
non-identical in reference, with those in (3) and (4), 
where they are non-overlapping in reference. In (1) 
and (2), "he" and "him" cannot refer to "John" (non- 
identical reference); while in (3) and (4) "John" cannot 
be a member of the set referred to by "they" and 
"them" (non-overlapping or disjoint reference). 
(1) He likes John. (3) They like John. 
(2) John likes him. (4) John likes them. 
Disjoint reference is even more noticeable with 
first and second person pronouns where it does not 
merely produce impossible interpretations, but actual 
ungrammaticality: 
(5) *1 like me. (7) "We like me. 
(6) "i like us~ (8) °Yo--'u like yo---u. 
A crucial notion both for assigining antecedents to 
bound anaphors and for establishing disjoint refer- 
ence between Noun Phrases is that of c-command, a 
structural relation. Briefly, a node c-commands its 
sisters and any nodes dominated by its sisters? 
Figure 2-1 illustrates this. 
A B 
C I 
E F 
A c-commands B, C, F, D, and G 
B c-commands A and E 
C c-commands D and G 
D c-commands C and F 
D I 
G 
Figure 2-1: C-Command 
IThis differs from Roinhart's (1976) definition, for reasons dis- cussed in Section 6. 
262 
Essentially, the relation between c-command and 
reference phenomena is the following: 
1. A non-pronominal NP cannot overlap in 
reference with any NP that c-commands it. 
2. The antecedent of a bound anaphor must 
c-command it. 2 
3. A personal pronoun cannot overlap in ref- 
erence with an NP that c-commands it. 2 
Condition 1 is motivated by sentences such as 
those in (9), where the underlined pronouns "he", 
"him", "they", and "them" must be disjoint in refer- 
ence with "John". In each case, the pronouns c- 
command the NP "John". In (ga) "he"/"they" is in the 
subject position, and so c-commands "John", in the 
direct object slot. In (gb) the pronouns ("He", "They") 
are once again in the subject position, and "John" is 
the object of a preposition, itself contained in the 
direct object of the sentence. Finally, in (9c), the NP 
"John" appears as the object of a preposition, which 
is o-commanded by the subject ("He", "They") and 
the direct object ("him", "them"). 
(9) a. He likes John. 
They' like John. 
b. He likes pictures of John. 
They' like pictures of John. 
c. He told them about John. 
They' told him about John. 
Condition 2 is motivated by examples such as 
those in (10), where the reflexive pronoun "himself" 
and its antecedent(s) are bracketed. As in the cor- 
responding examples in (9), "himself" either appears 
as a direct object (10a), the object of a preposition 
within the direct object (10b), or as a prepositional 
object (10c). In all cases, the c-commanding subject 
("John") is a possible antecedent; in (10c), where the 
c-commanding object NP "Bill" is added, it is also a 
possible antecedent. 
(10) a. \[John\] likes \[himself\]. 
b. \[John\] likes pictures of \[himself\]. 
c. \[John\] told \[Bill\] about \[himself\]. 
Condition 3 is motivated by examples such as 
those in (11). The pronoun under consideration 
("him" or "them") always appears as an object or 
prepositional object and is disjoint in reference to the 
c-commanding subject "John" (in (1 la,b,c)) and to the 
c-commanding direct object "Bill" in (1 lc). 
(11) a. John likes him. 
John likes them. 
b. John likes pictures of him. 
John likes pictures of them. 
c. John told Bill about him. 
John told Bill about them. 
While condition 1 is unconditionally true, con- 
ditions 2 and 3 are subject to a further constraint, 
=Within a minimal syntactic domain; this will be explained shortly, 
which we might term minimafity. Essentially, the 
structural theory of pronominal reference outlined 
here may be viewed as making the following claim. 
Bound anaphors are short-distance anaphors and re- 
quire their antecedents to be c-commanding NPs 
within a minimal domain. Ordinary personal 
pronouns, on the other hand, are long-distance 
anaphors, and only permit antecedents to come from 
outside of their minimal domain, and exclude any c- 
commanding antecedents within their minimal 
domain. The most immediately dominating finite 
clause (S) node always constitutes a minimal domain 
for a bound anaphor or personal pronoun. NP nodes 
normally do not constitute a minimal domain, unless 
they contain a possessive. This is illustrated in 
(12)--(14) (underlining indicates disjoint reference: 
bracketing indicates co-reference). The subject NP in 
(13) is not a possible antecedent for the reflexive; 
while the subject NP in (14) need not be disjoint in 
reference with the underlined pronoun. Compare (13) 
with (10b) and (14) with (1 lb). 
(12) He likes Bill's pictures of John. 
They' like Bill's pictures of John. 
(13) John likes \[Bill's\] pictures of \[himself\]. 
(14) \[John\] likes Bill's pictures of \[him\]. 
\[John\] likes Bill's pictures of \[them\]. 
Given these paradigms of reference facts, we now 
turn to the theoretical linguistics literature for treat- 
ments that might be implemented in a natural lan- 
guage system. In the Government-Binding framework 
of Chomsky (1981), these generalizations are cap- 
tured by the Binding Theoryma set of well- 
formedness conditions on syntactic structural 
representations annotated with subscript and super- 
script "indices". The paradigm assumed there is 
Generate and Test: indices are freely assigned and 
the Binding Conditions are applied to rule in or rule 
out a particular assignment. Clearly, from a computa- 
tional standpoint this is grossly inefficient. However, 
in earlier work, Chomsky (1980, pp. 38--44) proposed 
a two pass indexing mechanism that captures these 
facts procedurally. 
His proposal assigns each non-bound anaphor 
(i.e. non-pronominal NP or personal pronoun) the pair 
(r,A) where r (for Referential index) is a non-negative 
integer and A (for Anaphoric index) is a set of such 
integers. In the first pass, r and A are assigned from 
left-to-right in a depth-first manner. Each non-bound 
anaphor NP is assigned a unique r; in addition, the r 
index of each NP c-commanding it is added to its A 
index. This set of indices indicates all the other NPs 
with which it is disjoint in reference. For non- 
pronominal NPs, only one pass is needed: 
(15) John 2 told Bi11(3,{2} ) about Fred(4,{2.3} ) 
The indices here indicate that "John", "Bill", and 
"Fred" are all disjoint in reference. 
In the case of personal pronouns, a second pass 
is necessary. Consider example (14), repeated here 
as (16), after the first pass: 
263 
(16) John 2 likes Bill's(3,{2} ) pictures of him(4,{2,3} ) 
The indexing at this stage indicates that "Bill" is dis- 
joint in reference from "John" and that "him" is dis- 
joint in reference from "Bill", which is correct, and also 
from "John", which is not. To correct this, Chomsky 
(1980, pp. 38--44) has a second pass, in which the r 
indices of NPs outside the current minimal domain are 
removed from the A index of personal pronouns, 
thereby allowing them to serve as potential antece- 
dents. After this second pass, the indexing is: 
(17) John 2 likes Bilrs(3.(2} ) pictures of him(4.{3} ) 
At this stage "John" is no longer specified as being 
disjoint in reference with "him". 
We have taken this procedure as the basis for a 
more efficient pronominal reference algorithm that im- 
proves on two problematic features. First, while 
Chomsky's procedure requires two passes, our algo- 
rithm is single pass. While there may not be a great 
computational loss in the two-pass character of 
Chomsky's original proposal, clearly it is cleaner to do 
things in one pass. Moreover, the mechanism is ex- 
tensionally richer than Chomsky's: it also handles 
cases of backwards-pronominalization and split- 
antecedence. 
A second problem with Chomsky's procedure is 
that the potential antecedents of a personal pronoun 
are only implicitly represented: any NP whose r index 
is not a member of that pronoun's A index set is a 
syntactically permissible antecedent, but this set of 
permissible antecedents is not enumerated. For ex- 
ample, in (17), "John" is indicated as a potential an- 
tecedent of "him" by virtue of the fact that its r index, 
2, is not part of the A index of "him", and in no other 
way. Our algorithm explicitly indicates the potential 
antecedents of a personal pronoun. Again, this is 
more desirable than leaving this information implicit; 
besides the potential (and perhaps small) computa- 
tional savings of not needing to recompute this infor- 
mation, there is the more general consideration that 
we are not interested in creating syntactic represen- 
tations for their own sakes, but to make use of them. 
Explicitly representing antecedence information for 
personal pronouns contributes to this goal. 
In the next section, we show how our algorithm 
overcomes these limitations. 
3. THE ALGORITHM 
Before giving the details of the algorithm, we will 
sketch its general structure. The algorithm applies to 
a completed parse tree and traverses it in a left-to- 
right, depth-first manner. The algorithm uses the no- 
tion of minimal domain introduced in the preceding 
section: the S node or NP node (when minimality has 
been induced by the presence of a possessive) that 
most immediately dominates the node being 
processed, and the related notions of "internal" and 
"external" nodes. Internal nodes are dominated by 
the current minimal domain node; external nodes c- 
command the current minimal domain node. Essen- 
tially, the algorithm passes each node all the nodes 
that c-command it, subdivided into two sets, those 
that are internal to the current minimal domain and 
those that are external. As each node is processed, a 
subroutine is called that dispatches on the category of 
the node and performs any actions that are ap- 
propriate. It is this subroutine that implements the 
pronominal reference mechanism proper. 
Given this overview, we can now turn to the data 
structures that are used by the algorithm, as well as to 
the details of the algorithm. Each node in a parse tree 
is a Common LISP structure; two of its slots are used 
for establishing pronominal reference: 
:possible-antecedentsma list of all the nodes that 
can be co-referent or overlapping in reference with it. 
:lmpossible-antecedentsBa list of all the nodes 
that are disjoint in reference with it. 
The algorithm also uses two global 
variablesB*table-of-proforms* and 
*table-of-antecedents*rain a "blackboard" fashion. 
The algorithm uses two major procedures. The 
first, pass-down-c-commanding-nodes, is respon- 
sible for actually traversing each node in the tree. 
The actual algorithm it uses is shown in Figure 6-1 in 
a LISP-type notation. Its functionality can be stated 
as follows. Whenever it encounters a new node, it 
first processes that node by calling the procedure 
update-node, which will be described shortly. It next 
determines whether the node being processed counts 
as a minimal domain for its children. When the node 
is a finite S node, it does count as a minimal domain, 
for all its children. Hence, only nodes that it 
dominates can be internal nodes for its children; all 
other nodes are now treated as external by its 
children. When the node is an NP, there are two 
possibilities. If there is no possessive NP, the NP 
does not count as a minimal domain, hence, the ex- 
ternal nodes remain as before and the nodes it 
dominates are added to the set of internal nodes. 
However, when the NP does contain a possessive, it 
does count as a minimal domain, for all the nodes that 
it dominates, except the possessive itself. 3 Finally, if 
the node is of any other category, it is not a minimal 
domain, so the external nodes remain as before and 
the internal nodes are augmented by the constituents 
it dominates. 4 In all cases, 
pass-down-c-commanding-nodes calls itself recur- 
sively on the children of the node being processed, 
with the appropriate lists of internal and external 
nodes as arguments. 
update-node, in turn, processes the node passed 
~rhe reason for this exception will be explained in Section 6. 
4Non-finite clauses also need special treatment. However, con- 
sideration of this case requires discussion of whether non-finite 
clauses are Ss or VPs, which is beyond the scope of this paper. 
264 
to it, on the basis of the nodes internal and external to 
the current minimal domain. In particular, 
update-node performs the correct pronominal assign- 
ment. The algorithm used by update-node is shown 
in Figure 6-2 in a LISP-type notation. We also dis- 
cuss each clause separately. 
Clause \[I\] implements condition 1 (non-pronominal 
NPs). Since there are no minimality conditions on dis- 
joint reference for non-pronominal NPs, all NP nodes 
c-commanding a non-pronominal NP are added to its 
:impossible-antecedents slot, whether they are in- 
ternal (\[I.A\]) or external to the current minimal domain 
(\[I.B\]). This handles sentences such as those in (9) 
and (12). While it might seem odd to specify that a 
non-pronominal NP has no antecedents, this infor- 
mation is useful in handling cases of backwards 
pronominalization, as in (18). 
(18) \[His\] mother loves \[John\], 
Clause \[I.C\] handles backwards pronominalization by 
making use of information in °table-of-proforms*, a 
table of all the pronouns encountered so far in the 
course of the tree walk. s After update-node has 
added all c-commanding NP nodes to the 
:impossible-antecedents slot of a non-pronominal 
NP, it then searches *table-of-proforms* for any 
pronouns that are not on its 
:impossible-antecedents list; whenever it finds one, 
it adds the current non-pronominal NP to the 
pronoun's :possible-antecedents list. The last thing 
update-node does in processing a non-pronominal 
NP is to add it to *table-of-antecedents* (\[I.D\]), 
whose use will be explained shortly. 
Clause \[11\] implements condition 2 (bound 
anaphors). Since bound anaphors are short-distance 
anaphors, all and only the c-commanding NPs internal 
to the current minimal domain are added to the 
:possible-antecedents slot of a bound anaphor. 
Clause \[111\] implements condition 3 (personal 
pronouns). Since personal pronouns are long- 
distance anaphors, clause \[111\] performs a number of 
operations. First, all the c-commanding NPs internal 
to the current minimal domain are added to the 
:impossible-antecedents slot of a personal pronoun 
(\[Ill.A\]), disallowing them as antecedents. Next, all 
the c-commanding NPs external to the current min- 
imal domain are added to the :possible-antecedents 
slot of a personal pronoun (\[Ill.B\]), indicating that they 
are potential antecedents. Clause \[Ill.C\] handles sen- 
tences like (19). 
(19) \[John's\] mother loves \[him\]. 
in which a non-pronominal NP that does not c- 
command a personal pronoun serves as its antece- 
dent. As was noted above, each non-pronominal NP 
is added to the *table-of-antecedents* by clause 
\[I.D\]. When update-node has added all the ap- 
~'his lalok) is filled in by Clause \[Ill.D\]. 
propriate c-commanding nodes to the 
:impossible-antecedents slot of a personal pronoun, 
it then adds any NPs on *table-of-antecedents* that 
are not already on the pronoun's 
:impossible-antecedents slot to its 
:possible-antecedents slot. Finally, when 
update-node is finished processing a pronominal NP 
node, it adds it to *table-of-proforms (Jill.D\]), for use 
in backwards pronominalization. 
Note that, because our algorithm both establishes 
minimal domains and assigns possible and impossible 
antecedents during the course of the tree traversal, it 
can be single pass, in contrast to Chomsky's proce- 
dure, which assigned impossible antecedents in one 
traversal and checked for minimality during a second. 
Since update-node is a general mechanism for 
adding or modifying information to a node on the 
basis of c-commanding constituents it is fairly 
straightforward to extend to handle other phenomena 
that involve c-command by modifying its top level 
CASE statement to dispatch on other categories. In 
fact, we have extended it in this manner to handle 
examples of "N anaphora"; i.e. cases where the head 
noun of a Noun Phrase is either "one" (which has 
been argued in Baker (1978) to be an anaphor for Ns, 
i.e. a noun and its complements, but not for full Noun 
Phrases) or phonologically null (0), which seems to 
have the same possibilities for antecedents. 
(20) Give me a list of ships which are in the 
gulf of Alaska that have casualty reports 
dated earlier than Esteem's oldest one. 
(21) Is the Willamette's last problem rated 
worse than Wichita's 0? 
(when (p=o-n-bLr-p ofg-node) 
('loop for other-node 
e:Eke~na I -node-list 
(when (and (eqljal (category 
ot~,,e=-node ) 
(pEo-n-b=E-antecedent 
other-node) 
(add 
(get;-.,on-of-catego~ 
other-node ' N-~) 
(poeeible-anteoedmnt e 
ofg-nc~e) } ) ) ) ) 
Figure 3-1: Algorithm for Pro N-BAR 
Anaphora 
The addition to the algorithm that deals with this 
phenomenon is presented in Figure 3-1. This clause 
is considerably simpler that those that handle disjoint 
reference and co-reference phenomena for personal 
pronouns: only external nodes are involved and only 
forward antecedence is possible\] This c_lause finds all 
the Noun Phrases that c-command an N pro-form and 
that are external to the current minimal domain. This 
excludes the possessive in a Noun Phrase such as 
"Esteem's oldest one" or "Wichita's 9" from serving 
265 
(defun paee-down-o-oommanding-nodee (=fg-node external-node-list internal-node-list) 
(update-node ofg-nod@ external-node-liar internal-node-liar) 
(oond ((finite-olause ofg-node) 
(let ( (external-node-list (append internal-node-list exteEnal-node-liet) ) ) 
(loop for node in (Qhildren =fg-node) 
(let ((internal-node-list (eisters node) ) ) 
(paso-down- =- oommandlng-nodee node 
external -node - i i st 
internal-node-liar) ) ) ) ) 
( (equal (oategory ofg-node) 'NP) 
(mend ( (equal (oategoz~ (first (children ofg-node) ) ) 'NP) 
(pass-down-o-commanding-nodes (first (children ofg-node)} 
external-node- list 
internal-node- list } 
(let ( (external-node-list (append external-node-list internal-node-list) ) ) 
(loop for node in (feet (~!idren ofg-node)) 
(let ( (internal-node-list (eieteEs node) ) ) 
(poe • - down -o -o~-~anding-node • node 
external -node - li st 
internal-node-list) ) ) ) ) 
(T (loop for node in (c~hildren ofg-node) 
(let ((internal-node-liar (append (eietere node) internal-node-list))) 
(pas e - down-o -oc.~anding-node s node 
external-node- list 
internal-node-list) ) ) ) ) ) 
(T (loop for node in (children ofg-node} 
(let ((internal-node-list (append (sisters node) internal-node-list))) 
(paea-down-c-c~nanding-nodee node 
external-node-list 
internal-node-list) ) ) ) ) ) 
Figure 6-I: The Tree Walking Algorithm 
(dofun update-node (ofg-node external-node-list internal-node-list) 
(odes (oategory ofg-node) 
0~ 
(oond ( (non-pronomlnal ofg-node) \[~ 
(loop for other-node in @~ernal-node-liet \[I.A\] 
(when (equal (oltegozy other-node) 'NP} 
(add other-node (imposstble-antecedente ofg-node} ) ) ) 
(loop for other-node in internal-node-list \[\[.S\] 
(when (equal (category other-node) 'NP) 
(add other-node (4 .-Toeeible-anteoedente ofg-node) ) ) ) 
(loop for pro in *t&ble-of-proform~* \[\[.C\] 
(when (not (member pro (4~Doeei~le-anteoedente ofg-node) ) ) 
(add ofg-node (poeeible-anteoedente pro) ) ) } 
(push ofg-node *table-of-antecedents*) ) \[\[.D\] 
( (boond-enephor cfg-node) \[el\] 
(loop foe other-node in internal-node-list 
(when (equal (oatego=y other-node) 'NP) 
(add other-node (possible-antecedents ofg-node) ) ) ) ) 
( (personal-pronoun ofg-node) \[II~ 
(loop for other-node in internal-node-list \[lit.A\] 
(when (equal (oetegory other-node) 'NP) 
(add other-node (i .-Toeeible-anteoedente ofg-node) ) ) ) 
(loop foe other-node in external-node-list \[lll.B\] 
(when (equal (oategoEy other-node} 'NP) 
(add other-node (poeeible-antenedenta ofg-node) ) ) ) 
(loop foe NP in *table-of-anteoedentee \[Ill.C\] 
(when (not (member NP (4-.-~oeeible-antecedente ofg-node))) 
(add NI) (possible-antecedents ofg-node) ) ) ) 
(push ofg-node *table-of-profo===*) ) ) ) ) ) \[lU.D\] 
Figure 6-2: The Reference Algorithm 
266 
m 
as the antecedent to its pro-N. External NPs that 
meet this criterion are filtered, since not all NPs can 
be antecedents of an N anaphor. For example, 
proper nouns cannot serve as such antecedents. 
Each NP that meets these criteria has its N-BAR 
added to the :possible-antecedents slot of the N- 
BAR node being processed. 
4. INTERACTION WITH SEMANTIC 
INTERPRETATION 
Syntactic constraints will not always identify just 
one allowable referent for a pronoun. Consider (22): 
(22) The committee awarded the prize to itself. 
Syntactically, "itself" in this sentence can refer to ei- 
ther "the prize" or "the committee". The additional 
use of semantic constraints is required to determine 
that the proper referent of the reflexive pronoun is 
"the committee". 
Applying such constraints is the responsibility of 
the semantic interpretation component of our system. 
In the current implementation reported on here, 
semantic interpretation is applied after both parsing 
and the c-command tree-traversal have been per- 
formed. It is a two-stage process in which the first 
stage is concerned with "structural semantics"nthe 
semantic consequence of syntactic structurenand 
the second stage with "lexical semantics"~the 
specific meanings of individual words with respect to a 
given application domain. This architecture for 
semantic interpretation was adopted from the 
PHLIQA1 system (Bronnenberg, et al. (1980)) and 
has been used in ~'eating several difficult semantic 
phenomena (de Bruin and Scha (1988); Scha and 
Stallard (1988)). 
The structural semantics stage operates on the 
parse tree to produce an expression of a language 
called "EFL" (for English-oriented Formal Language). 
This language is a higher-order intensional logic 
which includes a single descriptive constant for each 
word in the lexicon, however many senses that word 
may have. (From this standpoint, therefore, EFL is 
actually an ambiguous logical language.) Expres- 
sions of EFL are produced from the parse tree by a 
system of semantic rules, paired one-for-one with the 
syntactic rules of the grammar, which compute the 
EFL translation of a tree node from the EFL trans- 
lations of its daughter nodes. The single EFL of a 
word is stored in its entry in the lexicon. 
The lexical semantics stage operates on an ex- 
pression of EFL to produce zero or more expressions 
of a language called "WML" (for World Model 
Language). WML is a higher-order intensional logic, 
with the same set of operations as EFL, but with un- 
ambiguous descriptive constants which correspond to 
the primitive concepts and relations of the particular 
application domain. WML expressions also have 
types, which are derived from the primitive disjoint 
categories of the application domain and which serve 
to delimit the set of meaningful WML expressions. 
A set of translation rules pair ambiguous con- 
stants of EFL with one or more unambiguous expres- 
sions of WML. Translation to WML is performed by 
producing all possible combinations formed from 
replacing the EFL constants with their translations, 
and filtering to remove combinations which are dis- 
allowed by WML's type system. In this way selec- 
tional restrictions are represented and enforced. 
The algorithms for producing EFL and WML are 
slightly modified in the case of anaphoric consituents: 
that is, reflexive pronouns, personal pronouns, and 
pro N-BARs~ When the structural semantics com- 
ponent encounters an anaphoric constituent in the 
course of translating a parse tree to EFL, it creates a 
new EFL constant "on the fly" to serve as the EFL 
translation of this constituent. It marks this constant 
specially and attaches to it the EFL translations of the 
syntactically possible antecedents of the constituent, 
along with semantic type information (such as for 
gender) constraining the antecedents which make 
sense for it. If the constituent is a personal pronoun 
or pro N-BAR (but not a reflexive pronoun), a special 
constant of WML is also attached, marked with the 
EFL translations of the impossible antecedents of the 
constituent. This special WML constant represents 
the possibility of extra-sentential resolution of the 
anaphor. 
The EFL to WML translation algorithm treats the 
anaphoric EFL constant specially, returning as its 
WML translations the translations of the "possible 
antecedents" that were attached in the EFL phase, 
together with the WML constant for extra-sentential 
reference (when this is appropriate). Expansion and 
filtering then proceed as described above. 
(22) is handled as follows. We will suppose the 
following "domain model" of WML constants and 
types: 
AWARD: (FUN (TUPLES AGENTS 
VALUABLES AGENTS) 
TV) 
SUB-TYPE(COMMITTEES,AGENTS) 
SUB-TYPE(PRIZES,VALUABLES) 
TYPE-INTERSECTION(VALUABLES,AGENTS) 
- NULL-SET 
The structural semantics stage constructs the fol- 
lowing clausal interpretation in EFL: 
(AWARD (THE COMMITTEES) (THE PRIZES) 
ITSELF001 ) 
where 
ITSELF001 --~ (THE COMMITTEES) 
(THE PRIZES) 
267 
The combinatorially possible WML translations are the 
following, where anomally with respect to the type 
system is marked with a ...... 
* (AWARD (THE COMMITTEES) (THE PRIZES) 
(THE PRIZES)) 
(AWARD (THE COMMITTEES) (THE PRIZES) 
(THE COMMITTEES)) 
The first interpretation is anomalous because the 
function "AWARD" is applied to an argument whose 
type is disjoint with the function's domain (in the third 
argument place). It is therefore discarded, leaving the 
second interpretation as the correct one. 
A different example; in which a pronoun could 
have an extra-sentential antecedent, is: 
(23) The committee awarded the prize to it. 
In this case, neither NP inside the sentence is syntac- 
tically allowable as an antecedent of "it", and so only 
the extra-sentential possibility remains. The WML 
translation for (23) is: 
(AWARD (THE COMMITTEE) (THE PRIZES) iT001) 
where IT001 is a WML constant marked for disjoint 
reference: 
IT001 ~ (THE COMMITTEES) 
(THE PRIZES) 
This information is necessary so that the module 
responsible for extra-sentential discourse can prevent 
external resolution of the pronoun to an internally 
(syntactically) forbidden antecadent--as could other- 
wise happen if "the committee" or "the prize" was 
mentioned in preceding discourse. 
Unless the anaphoric constituent is a reflexive 
pronoun, an extra-sentential alternative will always be 
present as a WML translation option, and survive type 
filtering (since it is given the most general possible 
type). When both intra- and extra-sentential alter- 
natives survive type filtering, our current heuristic is to 
prefer the intra-sentential one. 
5. COMPARISON WITH RELATED WORK 
Hobbs (1978) has done the only previous work we 
know of to use traversal of a syntactic parse tree to 
determine pronominal reference and we compare our 
algorithm with his in this section. Hobbs proposes a 
syntactic tree-traversal algorithm for pronominal refer- 
ence that is "part of a larger left-to-right interpretation 
process" (Hobbs (1978, p. 318)). When a pronoun is 
encountered, the algorithm moves up to the nearest S 
or NP node (our "minimal domain nodes") that 
dominates the pronoun and searches to the left of the 
pronoun for any NP nodes that are dominated by an 
intervening $ or NP node to propose as antecedents. 
The algorithm then proceeds up to the next NP or S 
node and searches to the left of the pronoun for any 
NP nodes to propose as antecedents. At this level, 
search is also made to the right for NP nodes to 
propose as antecedents. This will handle cases of 
backwards pronominalization, as in (18). However, 
this portion of the search is bounded; it does not seek 
antecedents below any NP or S nodes encountered. 
The search for c-commanding antecedents and an- 
tecedents for backwards pronominalization continues 
in this fashion until the top S is reached. At this point, 
preceding utterances in the discourse are searched, 
going from most recent to least recent. Each tree is 
searched in a left-to-right, breadth-first manner for 
NPs to propose as antecedents. 
There are several differences between this a{go- 
rithm and ours. The major one is that our algorithm is 
a single-pass, depth-first, exhaustive traversal 
whereas Hobbs' algorithm first walks down the tree, 
then up, and then back down and is not guaranteed to 
be exhaustive. Hobbs also imposes a "nearness" 
condition on the search for antecedents in the case of 
backwards pronominalization. However, as Hobbs 
points out, this restriction rules out the perfectly ac- 
ceptable (24a) and (24b). 
(24) a. Mary sacked out in \[his\] apartment before 
\[Sam\] could kick her out. 
b. Girls who \[he\] has dated say that \[Sam\] is 
charming. 
These examples show that the question of what the 
correct nearness constraint, if any, is remains open. 
Finally, Hobbs' algorithm handles both intra-sentential 
and extra-sentential pronominal reference relations, 
while ours is only intended to handle intra-sentential 
cases. 
6. CURRENT STATUS AND FUTURE 
RESEARCH 
In this section, we conclude by discussing some of 
the strengths and weaknesses of the current im- 
plementation and areas for future research. The 
shortcomings fall into two general categories: limita- 
tions of the implementation proper and limitations of 
the theory of pronominal reference that was imple- 
mented. 
There are two general sorts of limitations to the 
mechanism described here: those that may be over- 
come by adding additional filtering devices to the 
basic tree-walking engine and those that may require 
a change in that basic engine. We begin with limita- 
tions of the first sort. 
Currently, the algorithm does not do any checking 
on the potential antecedents of a pronoun or bound 
anaphora to see if they agree in person and number, s 
For bound anaphors, this is straightforward: a bound 
anaphor and its antecedent must agree in person and 
number. For personal pronouns, on the other hand, 
eCuwently, NPs are not specified for gender in our system, so this 
cannot be checked. 
268 
the situation is more complicated. In the singular, first 
('T', "me"), second ("you"), and third ("he", "him", 
"she", "her", "it") personal pronouns require agree- 
ment in both person and number. In the plural, 
however, the number requirement is dropped because 
of "split antecedents" cases, in which more than one 
NP forms part of the antecedent of a pronoun, as in: 
(25) \[John\] told \[Bill\] that \[they\] should leave. 
where "John" and "Bill", together, antecede "they". 
Third person plural pronouns still require that each 
antecedent of a split antecedent itself be third person. 
First person ("we", "us") and second person 
("you") pronouns also allow split antecedents, but 
with looser person agreement requirements: 
(26)a. \[I\] told \[John\] that \[we\] should go. 
b. \[I\] told \[you\] that \[we\] should go. 
c. \[Bill\] told \[you\] that \[you\] should go. 
d. I told \[you\] that \[you\] should go. 
e. ~John told __Bill that w._.ee should go. 
f. John told Bil.._ll that you should go. 
Note that a first person plural pronoun allows split 
antecedents only if at least one of them is itself first 
person; contrast (26a) and (26b) with (26e). Similarly, 
a second person plural pronoun allows split antece- 
dents only if at least one of them is also second 
personmcontrast (26c) with (26f)--but not if one is 
first person; contrast (26c) with (26d). 
While the constraints on singular and third person 
plural pronouns could be implemented as a local 
agreement check (e.g. as a pre-condition for being 
added to a pronoun's :possible-antecedents slot), 
the person agreement constraint on first and second 
person plural pronouns would require a separate post- 
process, since it is not a local constraint on individual 
split antecedents, but a global constraint on the set of 
them. Currently, since our algorithm imposes no 
agreement checks, it allows both the good cases of 
split antecedents as well as the impossible ones. We 
need to add the check to our algorithm and extend the 
semantics to also deal with split antecedents. 
The algorithm also does not check for "crossover" 
cases. Roughly speaking, these are examples similar 
to backwards pronominalization cases such as (18) 
(repeated here as (27a)), in which the potential an- 
tecedent is a quantifier or a trace of a moved WH 
element. In such cases, overlapping reference is im- 
possible. Contrast (27a) with (27b) and (27c). 
(27) a. \[His\] mother loves \[John\]. 
b. ~His mother loves everyone. 
c. Who does hi._? mother love twho? 
These particular cases can be handled by adding 
a check to clause \[I.C\] to prohibit quantified NPs and 
WH-traces from participating in backwards 
pronominalization. However, the more general 
problem of how elements dislocated by WH move- 
ment or by topicalization interact with the algorithm 
given here is a topic that requires further work beyond 
this simple measure. 
More seriously, there is also a well-known case of 
pronominal reference within NPs that is not handled 
by the algorithm. A constraint from the syntactic 
theory of reference implemented by our algorithm is 
that if the antecedent-anaphor relation holds between 
two positions, disjoint reference also holds between 
them; see examples (10) and (11), and (13) and (14). 
However, there is one position in English where this 
generalization is known not to hold: the possessive 
position of an NP. A bound anaphor is possible here, 
but a pronoun in the same position is not subject to 
disjoint reference; see (28): 
(28) a. \[The men\] read \[each other's\] books. 
b. \[The men\] read \[their\] books. 
(28a) is correctly handled by the algorithm as al- 
ready outlined; pass-down-c-commanding-nodes 
treats the nodes internal to the current minimal 
domain as internal nodes for the possessive in a 
Noun Phrase, so the NP "the men" will be added to 
the :possible-antecedents slot of a bound anaphor 
in this position. However, the same characteristics of 
the algorithm will also result in the NP "the men" be- 
ing assigned to the :impossible-antecedents slot of 
"their" in (28b). One possible remedy for this situa- 
tion is to add a clause to update-node that checks for 
possessive pronouns separately from other pronouns 
and that allows NPs both internal and external to the 
current minimal domain to be possible antecedents. 
However, the more far-reaching modifications 
proposed in the discussion below of the theory of 
pronominal reference would obviate this change. 
There are several areas where our implemen- 
tation points out problems with the structural theory of 
pronominal reference. The first of these is the defini- 
tion of c-command itself. 7 Under Reinhart's (1976) 
original definition, a node A c-commands node B iff 
the branching node most immediately dominating A 
also dominates B and A does not dominate B. The 
difference between the two definitions can be seen in 
Figure 2-1; in addition to the c-command statements 
given there, Reinhart's definition adds the following: 
E c-commands B, C, F, D, and G 
F c-commands D and G 
G c-commands C and F 
These statements are true under Reinhart's definition 
of c-command, because no branching category inter- 
venes between the c-commanding and c-commanded 
nodes, but not under that used in the implemented 
algorithm, since there is no sisterhood among the 
nodes. We have found this modified definition to be 
easier to implement; moreover, various researchers 
(e.g. Aoun and Sportiche (1983)i have pointed out 
problems with Reinhart's definition that the modified 
definition solvas. 
7Our algorithm uses a definition that is equivalent to the in 
co~ttuction with relation of Klima (1964, p. 297), which inspired c-command. 
269 
The implementation has also brought to light 
asymmetries in the strictness of c-command used to 
determine the antecedents of a bound anaphor and 
that used to determine the non-antecedents of a 
pronoun. In particular, none of the conjuncts of a 
conjoined NP can be the antecedent of a reflexive: 
(29) "John and Mary like himself. 
However, all of the conjuncts of a conjoined NP are 
impossible antecedents for any pronoun for which the 
entire conjoined NP is an impossible antecedent. In 
(30) John and Mary like him. 
"John" cannot be an antecedent of "him", despite the 
fact that "John" does not c-command "him". Contrast 
this with (19) where a non-c-commanding possessive 
can be the antecedent of a pronoun. This is handled 
correctly in the implementation. Whenever our algo- 
rithm adds a conjoined NP to the 
:impossible-antecedents slots of a pronoun or a 
non-pronominal NP, it adds all the conjuncts of that 
NP, as well. While this works, there is clearly some- 
thing that is being missed here. Presumably, it should 
follow by definition that no individual conjunct of a 
conjoined NP can be a possible antecedent of a Noun 
Phrase with which the entire conjoined NP is disjoint 
in reference, s 
A more serious problem with the theory of 
pronominal reference elaborated in Chomsky (1980) 
and (1981), and which our algorithm implements, is 
the crucial assumption that referentially dependent 
Noun Phrases can be exhaustively partitioned into 
bound anaphors vs. personal pronouns and that, 
therefore, they will be in complementary distribution. 
However, examples such as (28), as well as (31) 
(pointed out by Kuno (1987)) and (32) indicate that 
the notion of exhaustive partitioning of bound 
anaphors against personal pronouns is incorrect in 
the general .case, even though it may be the typical 
state of affairs. 
(31) a. \[John\] put the blanket under \[himself\]. 
b. \[John\] put the blanket under \[him\]. 
(32) a. Ill buy myself a beer. 
b. rll buy me a beer. 
We can keep the insight of the structural theory of 
pronominal reference (i.e. that structural relations play 
a role in delimiting reference possibilities), while still 
incorporating these facts, if we give up the restriction 
that bound anaphors and personal pronouns are al- 
ways in complementary distribution. One possible ap- 
proach to this problem is to use feature decomposition 
to characterize bound anaphors and pronouns: the 
feature +short-distance indicates whether a 
pronominal can be used as a short-distance anaphor 
while the feature :l:long-distance indicates whether it 
eThanks to Leland George for this insight, as well as for discussion 
of short and long distance enephors. 
can be used as a long-distance anaphor. 9. While, in 
the normal case, personal pronouns in English are 
specified to be long-distance anaphors that cannot be 
used as short-distance anaphors, (i.e. as 
I-short-distance +long-distance\]) this system would 
allow the feature governing a pronominal's use as a 
short-distance anaphor to be left free (i.e. as 
?short-distance) in certain syntactic contexts in 
English, such as the possessive position of a Noun 
Phrase, the object of certain prepositions, and the in- 
direct object position of verbs. 1° Such a view of the 
syntax of personal pronouns could be implemented in 
a unification grammar fairly straightforwardly. 
While such a treatment of personal pronouns as 
short-distance anaphors does not handle all the 
counter-examples to the syntactic theory of 
pronominal reference raised by researchers such as 
Kuno, it does begin to address them seriously. 
Clearly, it is more in accord with the facts than a 
theory that postulates an exhaustive partitioning of 
bound anaphors vs. personal pronouns, and so con- 
stitutes, in our opinion, a promising start towards han- 
dling the full range of pronoun reference facts in a 
reasonable manner. 
Finally, we consider alternate ways of combining 
our pronominal reference mechanism with parsing 
and semantic interpretation. One possibility is a fully 
incremental architecture in which c-command con- 
straints, semantic interpretations, and external refer- 
ence resolution are computed simultaneously with the 
parse. Such an architecture might seem particularly 
attractive for processing large sets of alternatives, 
such as are encountered when processing spoken in- 
put. The intra-sentential reference phenomena 
described in this paper pose a problem for such an 
incremental approach, however. The possiblities for 
internal resolution for an anaphor cannot all be known 
locally to the anaphor, but must be obtained from 
elsewhere in the sentence. In many cases antece- 
dents will lie to the left of the anaphor in the sentence, 
and thus will have been seen by a left-to-right parser 
by the time the anaphor is reached. But consider a 
case of backward pronominalization, as in (18), 
repeated here as (33): 
(33) His mother loves John. 
A wholly incremental mechanism, parsing the NP "his 
mother" first, would have to conclude that the referent 
of "his" was extra-sentential, since no intra-sentential 
referent was seen to the left. And if no extra- 
sentential referent could be found, the NP would have 
to be rejected. To be successful, such an incremental 
mechanism would have to be modified to include a 
kind of "lazy evaluation" which could rule out certain 
~'hle is akin to the feature system :l:anaphod¢ :l:pronomlnal of 
Chomsky (1981) 
'°This suggestion was originally made by Lust, et aL (1989) who 
support it on the basis of language acquisition data. 
270 
referents for an anaphor but never rule an anaphor 
empty of referents until utterance processing had 
been completed. 
Another alternative would be to separate intra- 
sentential anaphor resolution from semantic inter- 
pretation, performing it instead in conjunction with 
extra-sentential discourse processing. A possible 
problem for this approach can be seen in sentences 
where the anaphor is combined with another am- 
biguous element, so that proliferation of semantic in- 
terpretations occur, as in: 
(34) John's car is better than Bill's. 
where the pro N-BAR, left completely unspecified 
during semantic interpretation, is free to generate all 
sorts of combinations with the possessive, including 
those in which the possession is appropriate to 
various "relational" interpretations of the pro N-BAR 
(de Bruin and Scha (1988)). 
In future work, we plan to combine parsing and 
semantic interpretation into a single unification gram- 
mar incorporating semantic information in additional 
features. Part of that work will be to look for the 
optimal method of combining it with the pronominal 
reference mechanism presented here. 
ACKNOWLEDGEMENTS 
The work reported here was supported by the Ad- 
vanced Research Projects Agency under Contract No. 
N00014-C-87-0085 monitored by the Office of Naval 
Research. The views and conclusions contained in 
this document are those of the author and should not 
be interpreted as necessarily representing the official 
policies, either expressed or implied, of the Defense 
Advanced Research Projects Agency of the United 
States Government. 
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