A Theory of Parallelism and the Case of VP Ellipsis 
Jerry R. Hobbs and Andrew Kehler 
Artificial Intelligence Center 
SRI International 
333 Ravenswood Avenue 
Menlo Park, CA 94025 
{hobbs, kehler}©ai, sri. com 
Abstract 
We provide a general account of parallelism 
in discourse, and apply it to the special 
case of resolving possible readings for in- 
stances of VP ellipsis. We show how seyeral 
problematic examples are accounted for in 
a natural and straightforward fashion. The 
generality of the approach makes it directly 
applicable to a variety of other types of el- 
lipsis and reference. 
1 The Problem of VP Ellipsis 
VP ellipsis has received a great deal of atten- 
tion in theoretical and computational linguistics 
(Asher, 1993; Crouch, 1995; Dalrymple, Shieber, 
and Pereira, 1991; Fiengo and May, 1994; Gawron 
and Peters, 1990; Hardt, 1992; Kehler, 1993; Lappin 
and McCord, 1990; Priist, 1992; Sag, 1976; Web- 
bet, 1978; Williams, 1977, inter alia). The area is 
a tangled thicket of examples in which readings are 
mysteriously missing and small changes reverse judg- 
ments. It is a prime example of a phenomenon at 
the boundary between syntax and pragmatics. 
VP ellipsis is exemplified in sentence (1). 
(1) John revised his paper before the teacher did. 
This sentence has two readings, one in which the 
teacher revised John's paper (the strict reading), and 
one in which the teacher revised his own paper (the 
sloppy reading). Obtaining an adequate account of 
strict/sloppy ambiguities has been a major focus of 
VP ellipsis research. This is challenging because not 
all examples are as simple as sentence (1). In fact, 
sentence (1) is the first main clause of one of the 
more problematic cases in the literature: 
(2) John revised his paper before the teacher did, 
and Bill did too. 
Whereas one might expect there to be as many as 
six readings for this sentence, Dalrymple et ai. (1991, 
henceforth DSP) note that it has only five readings; 
the reading is absent in which 
(3) John revised John's paper before the teacher 
revised John's paper, and Bill revised John's 
paper before the teacher revised Bill's paper. 
Previous analyses have either generated too few or 
too many readings, or have required an appeal to 
additional processes or constraints external to the 
actual resolution process itself. 
Examples like (2) test the adequacy of an analysis 
at a fine-grained level of detail. Other examples test 
the generality of an analysis, in terms of its ability 
to account for phenomena similar to VP ellipsis and 
to interact with other interpretation processes that 
may come into play. For instance, strict/sloppy am- 
biguities are not restricted to VP ellipsis, but are 
common to a wide range of constructions that rely 
on parallelism between two eventualities, some of 
which are listed in Table 1. Given the ubiquity 
of strict/sloppy ambiguities, one would expect these 
to be a by-product of general discourse resolution 
mechanisms and not mechanisms specific to VP el- 
lipsis. Any account applying only to the latter would 
miss an important generalization. 
In this paper, we give an account of resolution 
rooted in a general computational theory of paral- 
lelism. We demonstrate the depth of our approach 
by showing that unlike previous approaches, the al- 
gorithm generates the correct five readings for ex- 
ample (2) without appeal to additional mechanisms 
or constraints. We also discuss how other 'missing 
readings' cases are accounted for. We show the gen- 
erality of the approach by demonstrating its han- 
dling of several other examples that prove prob- 
lematic for past approaches, including a source-of- 
ellipsis paradox, so-called extended parallelism cases, 
and sloppy readings with events cases. Of the phe- 
394 
Phenomenon Example 
'Do It' Anaphora 
'Do So' Anaphora 
Stripping 
Comparative Deletion 
'Same As' Reference 
'Me Too' Phenomena 
'one' Anaphora 
Lazy Pronouns 
Anaphoric Deaccenting 
Focus Phenomena 
John revised his paper before Bill did it. 
John revised his paper and Bill did so too. 
John revised his paper, and Bill too. 
John revised his paper more quickly' than Bill. 
John revised his paper, and Bill did the same. 
John revised his paper, and the teacher followed suit. 
A: John revised his paper. 
B: Me too./Ditto. 
John revised a paper of his, and Bill revised one too. 
The student who revised his paper did better than 
the student who handed it in as is. 
John said he called his teacher an idiot, 
and Bill said he insulted his teacher too. 
Only John revised his paper. 
Table 1: Phenomena Giving Rise to Sloppy Interpretations 
nomena in Table 1, we briefly discuss the algorithm's 
handling of lazy pronoun cases. 
2 A Theory of Parallelism 
The Theory A clause conveys a property or even- 
tuality, or describes a situation, or expresses a 
proposition. We use the term "property" to cover 
all of these cases. A property consists of a predi- 
cate applied to a number of arguments. We make 
use of a duality between properties having a number 
of arguments, and arguments having a number of 
properties. Parallelism is characterized in terms of 
a co-recursion in which the similarity of properties is 
defined in terms of the similarity of arguments, and 
the similarity of arguments is defined in terms of the 
similarity of properties. 1 
Two fragments of discourse stand in a parallel re- 
lation if they describe similar properties. Two prop- 
erties are similar if two corresponding properties can 
be inferred from them in which the predicates are the 
same and the corresponding pairs of arguments are 
either coreferential or similar. 
Similarly(el,x1, .. •, Zl), p2(e2; x2, ..., z2)\]: 
p~(el,xl,...,Zx) ~ p'(el,xl,...,zl) and 
I e ..., ..., P2( 2,X2, Z2) Dp'(e2,x2, z2), where 
Corer(x1,..., x2 .... ) or Similar\[x1, x2\], 
Corer(z1,..., z2, . . .) or Similar\[z1, z2\] 
Two arguments are similar if their other, "inferen- 
tially independent" properties are similar. 
Similar\[xl, x2\]: 
Similar~ (..., zl, . . .),p~2 (..., x2, . . .)\], 
... 
Similar\[q~ (..., Xl , . . .), q~ (..., x2, . . .)\] 
1This account is a elaboration of treatments of par- 
allelism by Hobbs (1979; 1985) and Kehler (1995). 
The constructed mapping between pairs of argu- 
ments must be preserved and remain one-to-one. 
There are three ways the recursion can bottom 
out. we can run out of new arguments in prop- 
erties. We can run out of new, inferentially inde- 
pendent properties of arguments. And we can "bail 
out" of proving similarity by proving or assuming 
coreference between the two entities. 
Two properties are inferentially independent if 
neither can be derived from the other. Given a 
knowledge base K representing the mutual knowl- 
edge of the participants in the discourse, properties 
P1 and P2 are inferentially independent if neither 
K,/)1 I-- P~ nor K, P2 ~- PI. This rules out the case 
in which, for example, the fact that John and Bill 
are both persons would be used to establish their 
similarity when the fact that they are both men 
has already been used. Inferential independence is 
generally undecidable, but in practice this is not a 
problem. In discourse interpretation, all we usually 
know about an entity is the small set of properties 
presented explicitly in the text itself. We may take 
these to be inferentially independent and look for no 
further properties, once properties inferrable from 
these have been used in establishing the parallelism. 
Similarity is a matter of degree. The more corre- 
sponding pairs of inferentially independent proper- 
ties that are found, and the more contextually salient 
those properties are, the stronger the similarity. In 
a system which assigns different costs to proofs (e.g., 
Hobbs et al. (1993)), the more costly the proofs re- 
quired to establish similarity are, the less similar the 
properties or arguments should seem. Interpreta- 
tions should seek to maximize similarity. 
This account of parallelism is semantic in the sense 
that it depends on the content of the discourse rather 
than directly on its form. But syntax plays an im- 
plicit role. When seeking to establish the paral- 
395 
lelism between two clauses, we must begin with the 
"top-level" properties; this is generally determined 
by the syntactic structure of the clause. Then the 
co-recursion through the arguments and properties 
normally mirrors the syntactic structure of the sen- 
tence. However, features of syntax that are not man- 
ifested in logical form are not taken into account. 
An Example To illustrate that the theory has 
applicability well beyond the problem of VP ellip- 
sis, we present an example of semantic parallelism 
in discourse. It comes from an elementary physics 
textbook, and is worked out in essentially the same 
manner in Hobbs (1979). 
(4) A ladder weighs 100 lb with its center of grav- 
ity 10 ft from the foot, and a 150 lb man is 
10 ft from the top. 
We will assume "the foot" has been identified as the 
foot of the ladder. Because it is a physics problem, 
we must reduce the two clauses to statements about 
forces acting on objects with magnitudes in a direc- 
tion at a point in the object: 
force(w1, L, dl, zl); force(w2, y, d2, x2) 
In the second clause we do not know that the man 
is standing on the ladder--he could be on the roof-- 
and we do not know what "the top" is the top of. 
These facts fall out of recognizing the parallelism. 
The procedure for establishing parallelism is il- 
lustrated in Figure 1, in which parallel elements are 
placed on the same line. The force predicates are the 
same so there is no need to infer further properties. 
The first pair of arguments, wl and w2 are similar in 
that both are weights. To make the second pair of 
arguments similar, we can assume they are corefer- 
ential; as a by-product, this tells us that the object 
the man's weight is acting on is the ladder, and hence 
that the man is on the ladder. The third pair of argu- 
ments are both downward directions. The final pair 
of arguments, x~ and x2, are similar if their proper- 
ties distance(x1, f, 20ft) and distance(x2, t, 10ft) are 
similar. These will be similar if their previously un- 
matched pair of arguments f and t are similar. This 
holds if their properties foot(f, L) and top(t, z) are 
similar. We infer end(f, L) and end(t, z ), since feet 
and tops are ends. Finally, we have to show L and 
z are similar. We can do this by assuming they are 
coreferential. This, as a by-product, tells us that the 
top is the top of the ladder. 
The use of inferences, such as '% foot is an end", 
means that this theory is parametric on a knowl- 
edge base. Different sets of beliefs can yield different 
bases for parallelism and indeed different judgments 
about whether parallelism occurs at all. 
A crucial piece of our treatment of VP-ellipsis is 
the explicit representation of coreference relations, 
denoted with the predicate Core\]. We could use 
equalities such as y = L, or since equals can be re- 
placed by equals, simply replace y with L. However, 
doing this would lose the distinction between y and 
L under their corresponding descriptions. 
Consequently, we introduce the relation 
Corer(y, e~, x, el) to express this coreferentiality. 
This relation says that y under the description as- 
sociated with e2 is coreferential with x under the 
description associated with el. From this we can in- 
fer y = x but not e2 = el, and the coreferentiality 
cannot be washed out in substitution. A constraint 
on the arguments of Corefis that el and e2 be prop- 
erties of x and y respectively. 
The phenomenon of parallelism pervades dis- 
course. In addition to straightforward examples of 
parallelism like the above, there are also contrasts, 
exemplifications, and generalizations, which are de- 
fined in a similar manner. The interpretation of a 
number of syntactic constructions depends on recog- 
nizing parallelism, including those cited in Table 1. 
In brief, our theory of parallelism is not something 
we have introduced merely for the purpose of han- 
dling VP ellipsis; it is needed for a wide range of 
sentential and discourse phenomena. 
Other Approaches Based on Parallelism Our 
aim in this paper is to present the theory of paral- 
lelism at an abstract enough level that it can be em- 
bedded in any sufficiently powerful framework. By 
"sufficiently powerful" we mean that there must be 
a formalization of the notion of inference, strength 
of inference, and inferential independence, and there 
must be a reasonable knowledge base. In Hobbs and 
Kehler (forthcoming), we show how our approach 
can be realized within the "Interpretation as Ab- 
duction" framework (Hobbs et al., 1993). 
There are at least two other treatments in which 
VP ellipsis is resolved through a more general system 
of determining discourse parallelism, namely, those 
of PriJst (1992) and Asher (1993). 
Prfist (1992) gives an account of parallelism devel- 
oped within the context of the Linguistic Discourse 
Model theory (Scha and Polanyi, 1988). Parallelism 
is computed by determining the "Most Specific Com- 
mon Denominator" of a set of representations, which 
results from unifying the unifiable aspects of those 
representations and generalizing over the others. VP 
ellipsis is resolved as a side effect of this unifica- 
tion. The representations assumed, called syntac- 
396 
f orce(wl , L, dl, xl ) 
wl : lb(wl, 100) 
L : ladder(L) 
dl : Down(dl) 
xz : distance(xt, f, 20ft) 
f: foot(f, L) =~ end(f, L) 
L: 
force(w2, y, d~., z~.) 
w2 : lb(w2,150) 
y :~ Coref(y, ..., L, ...) 
d2 :Down(d2) 
x2 : distance(x2, t, 10ft) 
t : top(t, z) ~ end(t, z) 
z :~ Coref(z, ..., L, ...) 
Figure 1: Example of Parallelism Establishment 
tic/semantic structures, incorporate both syntactic 
and semantic information about an utterance. One 
weakness of this approach is that it appears overly 
restrictive in the syntactic similarity that it requires. 
Asher (1993) also provides an analysis of VP ellip- 
sis in the context of a theory of discourse structure 
and coherence, using an extension of Discourse Rep- 
resentation Theory. The resolution of VP ellipsis 
is driven by a need to maximize parallelism (or in 
some cases, contrast) that is very much in the spirit 
of what we present. 
Detailed comparisons with our approach are given 
with the examples below. In general, however, in 
neither of these approaches has enough attention 
been paid to other interacting phenomena to explain 
the facts at the level of detail that we do. 
3 VP Ellipsis: A Simple Case 
We first illustrate our approach on the simple case 
of VP ellipsis in sentence (1). The representation 
for the antecedent clause in our "logical form" ~ ap- 
pears on the left-hand side of Figure 2. Note that 
a Core\] relation links Xl, the variable corresponding 
to "he" (eventuality e13), to its antecedent j; the 
entity described by "John" (eventuality ell). 
From the second clause we know there is an elided 
eventuality e22 of unknown type P, the logical sub- 
ject of which is the teacher t. 
P(e22, t) 
t : teachert(e21, t) 
Because of the ellipsis, e22 must stand in a parallel 
relation to some previous eventuality; here the only 
candidate is John's revising his paper (e12). To es- 
tablish Similar(el2, e22),3 we need to show that their 
corresponding arguments are similar. John j and the 
2The normally controversial term "logical form" is 
used loosely here, simply to capture the information that 
the hearer must bear in mind, at least implicitly, in in- 
terpreting texts such as sentence (1). 
3 We cannot establish coreference between the events 
because their agents are distinct. In other cases, how- 
ever, the process can bail out immediately in event coref- 
erence; consider the sentence "John revised his paper, 
teacher t are similar by virtue of being persons. The 
corresponding objects Pl and/>2 are similar if we take 
p2 to be a paper and to have a Poss property similar 
to that of Pl. The latter is true if corresponding to 
the possessor Xl, there is an x2 that is similar to xl. 
In constructing the similarity between x2 and xl, 
we can either take them to be coreferential (case *a) 
or prove them to be similar by having similar prop- 
erties, including having similar dependencies estab. 
lished by Core\] (case *b). In the former case, x~ is 
coreferential with xl which is coreferential with John 
j, giving us the strict reading. In the latter case, we 
must preserve the previously-constructed mapping 
between John j (on which xl is dependent) and the 
teacher t; thus x2 is similar to xl if taken to be 
coreferential with t, giving us the sloppy reading. 4 
4 A Missing Readings Paradox 
Sentence (1) is the antecedent clause for example 
(2), one of the more problematic examples in the 
literature. Theoretically, this example could have as 
many as six readings, paraphrased as follows: 
(5) John revised John's paper before the teacher 
revised John's paper, and Bill revised 
John's/Bill's paper before the teacher revised 
John's/Bill's paper. 
(6) John revised John's paper before the teacher 
revised the teacher's paper, and Bill revised 
John's/Bill's paper before the teacher revised 
the teacher's paper. 
smoking incessantly as he did." A Core\] link is estab- 
lished between the elided and antecedent events in the 
same way as for pronouns. This symmetry accounts for 
another problematic case, discussed in Section 6. 
4It is also possible to "bail out" in coreference be- 
tween the papers pl and p2; here we would get the strict 
reading again. However, consider if the example had said 
"a paper of his" rather than "his paper". The resulting 
sentence has two strict readings, one in which both re- 
vised the same paper of John's (generated by assuming 
coreference between the papers), and one in which each 
revised a (possibly) different paper of John's (generated 
by assuming coreference between the pronouns). 
397 
before'(el2, e22) 
revise'(e12, j, Pl) 
j: John'(ell,j) 
Pl : paper'(els,pl) 
Poss'(e14, xl,pl) 
xl : he'(e13,xl) 
Coref(xl, el3, j, ell) 
revise'(e22, t, P2) 
t : teacher'(e21, t) 
P2 : papert(e25, P2) 
Poss' (e24, x2, P2) 
x2 : he'(e23,x2) 
\[Co~ef(z~., e23, xl, e13) (*a)\] 
\[Corel(z2, e23, t, e..,~) (*b)\] 
Figure 2: Representations for Simple Case 
We follow DSP in claiming that this example has five 
readings, in which the JJJB reading shown in (3) is 
missing. ~ DSP, who use this case as a benchmark 
for theories of VP ellipsis, note that the methods of 
Sag (1976) and Williams (1977) can be seen to derive 
two readings, namely JJJJ and JTBT. An analysis 
proposed by Gawron and Peters (1990), who first 
introduced this example, generates three readings 
(adding JJBB to the above two), as does the analysis 
of Fiengo and May (1994). A method that Gawron 
and Peters attribute to Hans Kamp generates either 
four readings, including the above three and JTJT, 
or all six readings. DSP's analysis strictly speak- 
ing generates all six readings; however, they appeal 
to anaphor/antecedent linking relationships to elim- 
inate the JJJB reading. However, these linking rela- 
tionships are not a by-product of the resolution pro- 
cess itself, but must be generated separately. Our 
approach derives exactly the correct five readings. 6 
The antecedent clause is represented in Figure 2, 
and the expansion of the final VP ellipsis is shown 
in Figure 3. In proving similarity, each pronoun can 
be taken to be coreferential with its parallel element 
(cases *a, *c and *e), or proven similar to it (cases 
*b, *d, *f and *g). If choice *a is taken in the sec- 
ond clause, then the "similarity" choice in the fourth 
clause must be *f; if *b, then *g. If *a and *c are 
chosen, the JJJJ reading results. If *a, *d, and *e 
are chosen, the JJBJ reading results. If *a, *d, and 
*f are chosen, the JJBB reading results. If *b and *c 
are chosen, the JTJT reading results. If *b and *d 
are chosen, the JTBT reading results. Thus taking 
all possible choices gives us all acceptable readings. 
Now consider what it would take to obtain the 
*JJJB reading. The variable x3 would have to be 
5Each reading for this example contains four descrip- 
tions of papers that were revised. We use the notation 
JJJB to represent the reading in which the first three 
papers are John's and fourth is Bill's, corresponding to 
reading (3). Other uses of such notation should be un- 
derstood analogously. 
6The approach presented in Kehler (1993) also derives 
the correct five readings, however, our method has ad- 
vantages in its being more general and better motivated. 
coreferential with John and x4 with Bill. The for- 
mer requirement forces us to pick case *c. But then 
case *e makes x4 coreferential with either John or 
the teacher (depending on how the first ellipsis was 
resolved). Case *f makes x4 coreferential with John, 
and case *g makes it coreferential with the teacher. 
There is no way to get x4 coreferential with Bill once 
we have set x3 to something other than Bill. 
Neither Prtist (1992) nor Asher (1993) discuss this 
example. In extrapolating from the analyses Pr/ist 
gives, we find that his analysis generates only two 
of the five readings. Briefly, if the first ellipsis is 
resolved to the strict reading, then the JJJJ read- 
ing is possible. If the first ellipsis is resolved to the 
sloppy reading, then only the JTBT reading is possi- 
ble. Asher's account, extrapolating from an example 
he discusses (p. 371), may generate as many as six 
readings, including the missing reading. This read- 
ing results from the manner in which the strict read- 
ing for the first ellipsis is generated--the final clause 
pronoun is resolved with the entity specified by the 
subject of the antecedent clause, whereas our algo- 
rithm creates a dependency between the pronoun 
and its parallel element in the antecedent clause. 
Our mechanism is more natural because of the align- 
ment of parallel elements between clauses when es- 
tablishing parallelism, and it is this property which 
results in the underivability of the missing reading. 
5 A Source-of-Ellipsis Paradox 
DSP identify two kinds of analysis in the VP ellip- 
sis literature. In identity-of-relations analyses (Sag, 
1976; Williams, 1977; Gawron and Peters, 1990; 
Fiengo and May, 1994, inter alia) strict/sloppy read- 
ings arise from an ambiguity in the antecedent VP 
derivation. The ambiguity in the ellipsis results 
from copying each possibility. In non-identity ap- 
proaches (Dalrymple, Shieber, and Pereira, 1991; 
Kehler, 1993; Crouch, 1995, inter alia) strict/sloppy 
readings result from a choice point within the reso- 
lution algorithm. Our approach falls into this class. 
Non-identity approaches are supported by exam- 
ples such as (7), which has reading (8). 
398 
before(e32, e42) 
revise' (e32, b, P3 ) 
b : Bill'(e31, b) 
p3 : paper'(e35, P3) 
P oss' ( e34 , x 3 , P3 ) 
x3 : he'(e33,x3) 
\[(*c) C,:,'ef(z3, e33, =~, e~3)\] 
\[(*d) Core.f (z3, e33, b, e31)\] 
Figure 3: Representations 
(7) John realizes that he is a fool, but Bill does 
not, even though his wife does. (Dahl, 1972) 
(8) John realizes that John is a fool, but Bill does 
not realize that Bill is a fool, even though 
Bill's wife realizes Bill is a fool. 
Example (7) contains two ellipses. Reading (8) re- 
sults from the second clause receiving a sloppy in- 
terpretation from the first, and the third clause re- . 
ceiving a strict interpretation from the second. An 
identity-of-relations analysis, however, predicts that 
this reading does not exist. Because the second 
clause will only have the sloppy derivation received 
from the first, the strict derivation that the third 
clause requires from the second will not be present. 
However, in defending their identity-of-relations 
approach, Gawron and Peters (1990) note that a 
non-identity account predicts that sentence (9) has 
the (nonexistent) reading given in (10). 
(9) John revised his paper before Bill did, but 
after the teacher did. 
(10) John revised John's paper before Bill revised 
Bill's paper, but after the teacher revised 
John's paper. 
In this case, the first clause is the antecedent for 
both ellipses. These two examples create a paradox; 
apparently neither type of analysis (nor any previous 
analyses we are aware of) can explain both. 
Our analysis accounts for both examples through 
a mutually-constraining interaction of parallelisms. 
Example (7) is fairly straightforward, so we focus on 
example (9). Let us refer to the clauses as clauses 1, 
2, and 3. Because clauses 2 and 3 are VP-elliptical, 
we must establish a parallelism between each of 
them and clause 1. In addition, the contrast rela- 
tion signalled by "but" is justified by the contrast- 
ing predicates "before" and "after", provided their 
corresponding pairs of arguments are similar. Their 
first arguments are similar since they are identical-- 
clause 1. Then we also have to establish the similar- 
ity of their second arguments--clause 2 and clause 3. 
revise' ( e42 , t, p4 ) 
t : teacher'(e41, t) 
P4 : paper'(e45,P4) 
Poss'(e44, x4, P4) 
x4 : he'(e4z, x4) 
\[Co~e/(z4, e43, z2, e~3) (*e)\] 
\[Core/(z4, e43, z3, e33) (*f)\] 
\[Co~el(x~, e,3, t, e,1) (*g)\] 
for Five Readings Case 
Thus, three mutually constraining parallelisms must 
be established: 1 - 2, 1 - 3, and 2 - 3. 
In Figure 4, cases *a and *b arise from the coref- 
erence and similarity options when establishing the 
parallelism between clauses 1 and 2, and cases *c 
and *d from the parallelism between clauses 1 and 
3. However, because parallelism is also required be- 
tween clauses 2 and 3, we cannot choose these op- 
tions freely. If we choose case *a, then we must 
choose case *c, giving us the JJJ reading. If we 
choose case *b, then we must choose case *d, giving 
us the JBT reading. Because of the mutual con- 
straints of the three parallelisms, no other readings 
are possible. This is exactly the right result. 
Prtist (1992) essentially follows Sag's (1976) treat- 
ment of strict and sloppy readings, which, like other 
identity-of-relations analyses, will not generate the 
reading of the cascaded ellipsis sentence (7) shown 
in (8). While the approach will correctly predict the 
lack of reading (10) for sentence (9), it does so for 
the wrong reason. Whereas ellipsis resolution does 
:not permit such readings in any circumstance in his 
account, we claim that the lack of such readings for 
• sentence (9) is due to constraints imposed by multi- 
ple parallelisms, and not because of the correctness 
of identity-of-relations analyses. 
Asher's (1993) analysis falls into the non-identity 
class of analyses, a~ld therefore makes the correct 
predictions for sentence (7). While he does not dis- 
cuss the contrast between this case and sentence (9), 
we do not see any reason why his framework could 
not accommodate our solution. 
6 Other Examples 
Missing Readings with Multiple Pronouns 
Dahl (1974) noticed that sentence (11) has only 
three readings instead of the four one might expect. 
The reading Bill said that John revised Bill's paper 
is missing. 
(11) John said that he revised his paper, and Bill 
did too. 
399 
before(el2, e22) 
e12 :revise'(e12,j, pl) 
j: John'(ell,j) 
Pl : paper'(e15,P1) 
Poss' (e14, xl, Pl) 
2;1 : he'(e13,x1) 
Co~ef(xl, el3, j, e11) 
after(el2, e32) 
e32 : revise'(e32, t,p3) 
t : teacher'(e31, t) 
P3 : paper~(e3s,P3) 
Poss' (e34, x3, P3) 
x3 : he'(e33,x3) 
\[Corer(x3, e33, Zl, el3) (*C)\] 
\[Corer(z3, e33, t, e31) (*d)\] 
e22 : revise' ( e22, b, p2 ) 
b : Billl(e21, b) 
P2 : paper' (e25, P2 ) 
Poss'(e24, x2, P2) 
x2 : he'(e23,x2) \[Co~e/(=2, 
e23, Zl, e13) (*a)\] 
\[Coref(x=, e23, b, e21) (*b)\] 
Figure 4: Representations for the Source-of-Ellipsis Paradox 
In contrast, the similar sentence given in (12) ap- 
pears to have all four readings. 
(12) John said that his teacher revised his paper, 
and Bill did too. 
The readings derived by our analysis depend on 
the Core\] relations that hold between the corefer- 
ring noun phrases in the antecedent clauses. For 
sentence (11), the correct readings result if his is 
linked to he and he to John; for sentence (12), the 
correct readings result if both pronouns are linked to 
John. Other cases in the literature indicate that the 
situation is more complicated than might initially be 
evident. Handling these cases requires an account 
of how such dependencies are established, which we 
discuss in Hobbs and Kehler (forthcoming). 
Extended Parallelism In some cases, the ele- 
ments involved in a sloppy reading may not be con- 
tained in the minimal clause containing the ellipsis. 
(13) John told a man that Mary likes him, and 
Bill told a boy that Susan does. ~ 
(14) The man who gives his paycheck to his wife 
is wiser than the man who gives it to his mis- 
tress. (Karttunen, 1969) 
the pronoun it does not refer to the first man's pay- 
check but the second's. 
In text, it normally requires an explicit, corefer- 
ring antecedent. However, the parallelism between 
the clauses licenses a sloppy reading via the similar- 
ity option.. The real world fact that to give some- 
thing to someone, you first must have it, leads to a 
strong preference for the sloppy reading. 
It is necessary to have parallelism in order to li- 
cense the lazy pronoun reading. If we eliminate the 
possibility of parallelism, as in 
(15) John revised his paper, and then Bill handed 
it in. 
the lazy pronoun reading is not available, even 
though the have-before-give constraint is not satis- 
fied. To interpret this sentence, we are more likely 
to assume an unmentioned transfer event between 
the two explicit events. 
Sloppy Readings with Events Sentence (16) 
has a "sloppy" reading in which the second main 
clause means "I will kiss you even if you don't want 
me to kiss you." 
(16) I will help you if you want me to, but I will 
kiss you even if you don't, s 
Deriving this reading requires a Core\] relation be- 
tween the elided event and its antecedent in the 
first main clause, which is obtained when our al- 
gorithm bails out in event coreference (see footnote 
8Mark Gawron, p.c., attributed to Carl Pollard. 
Although the antecedent clause for "Susan does" 
is "Mary likes him", there is a sloppy reading in 
which "Bill told a boy that Susan likes Bill". This 
fact is problematic for accounts of VP ellipsis that 
operate only within the minimal clauses. These 
readings are predicted by our account, as John and 
Bill are parallel in the main clauses. 
Lazy Pronouns "Lazy pronouns" can be ac- 
counted for similarly. In 
TThis example is due to Priist (1992), whose approach 
successfully handles this example. 
400 
3). Then in expahding the VP ellipsis in the sec- 
ond main clause, taking the similarity option for the 
event generates the desired reading. 
Inferentially-Determined Antecedents Web- 
bet (1978) provides several examples in which the 
antecedent of an ellipsis is derived inferentially: 
(17) Mary wants to go to Spain and Fred wants to 
go to Peru, but because of limited resources, 
only one of them can. 
Our account of parallelism applies twice in han- 
dling this example, once in creating a complex 
antecedent from recognizing the parallelism be- 
tween the first two clauses, and again in resolv- 
ing the ellipsis against this antecedent. Hobbs and 
Kehler (forthcoming) describe the analysis of this 
case as well as others involving quantification. 
7 Summary 
We have given a general account of parallelism in 
discourse and applied it to the special case of resolv- 
ing possible readings for instances of VP ellipsis. In 
doing so, we showed how a variety of examples that 
have been problematic for previous approaches are 
accounted for in a natural and straightforward fash- 
ion. Furthermore, the generality of the approach 
makes it directly applicable to a variety of other 
types of ellipsis and reference in natural language. 
Acknowledgements 
The authors thank Mark Gawron, David Israel, and 
three anonymous reviewers for helpful comments. 
This research was supported by National Science 
Foundation/Advanced Research Projects Agency 
Grant IRI-9314961. 

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