The Fitted Parse: 
100% Parsing CapabiliW in a Syntactic Grammar of English 
Karen Jensen and George E. Heidorn 
Computer Sciences Department 
IBM Thomas J. Watson Research Center 
Yorktown Heights, Hew York 10598 
Abstract 
A technique is described for performing fitted parsing. 
After the rules of a more conventional syntactic grammar are 
unable to produce a parse for an input string, this technique 
can be used to produce a reasonable approximate parse that 
can serve as input to the remaining stages of processing. The 
paper describes how fitted parsing is done in the EP\[STLE 
system and discusses how it can help in dealing with many 
difficult problems of natural language analysis. 
Introduction 
The EPISTLE project has as its long-range goal the ma- 
chine processing of natural language text in an office environ- 
ment. Ultimately we intend to have software that will be able 
to parse and understand ordinary prose documents (such as 
those that an office principal might expect his secretary to 
cope with), and will be able to generate at least a first draft of 
a business letter or memo. Our current goal is a system for 
critiquing written material on points of grammar and style. 
Our grammar is written in NLP (Heidorn 1972). an aug- 
mented phrase structure language which is implemented in 
LISP/370. The EPISTLE grammar currently uses syntactic, 
but not semantic, information. Access to an on-line standard 
dictionary with about 130.000 entries, including part-of-speech 
and some other syntactic information (such as transitivity of 
verbs), makes the system's vocabulary essentially unlimited. 
We test and improve the grammar by regularly running it on a 
data base of 2254 sentences from 411 actual business letters. 
Most of these sentences are rather complicated; the longest 
contains 63 words, and the average length is 19.2 words. 
Since the subset of English which is represented in busi- 
ness documents ,s very large, we need a very comprehensive 
grammar and robust parser. In the course of this work we 
have developed some new techniques to help deal with the 
refractory nature of natural language syntax. In this paper we 
discuss one such technique: the fitted parse, which guarantees 
the production of a reasonable parse tree for any string, no 
matter how unorthodox that string may be. The parse which is 
produced by fimng might not be perfect; but it will always be 
reasonable and useful, and will allow for later refinement by 
semantic processing. 
There is a certain perception of parsing that leads to the 
development of techniques like this one: namely, that trying 
to write a grammar to describe explicitly all and only the sen- 
fences of a natural language is about as practical as trying to 
find the Holy Grail. Hot only will the effort expended be 
Herculean, it will be doomed to failure. Instead we take a 
heuristic approach and consider that a natural language parser 
can be divided into three parts: 
(a) a set of rules, called the core grammor, that precisely 
define the central, agreed-upon grammatical structures 
of a language; 
(b) peripheral procedures that handle parsing ambiguity: 
when the core grammar produces more than one parse, 
these procedures decide which of the multiple parses is 
to be preferred; 
(c) peripheral procedures that handle parsing failure: when 
the core grammar cannot define an acceptable parse, 
these procedures assign some reasonable structure tO the 
input. 
In EPISTLE, (a) the core grammar consists at present of a set 
of about 300 syntax rules; (b) ambiguity is resolved by using a 
metric that ranks alternative parses (Heidorn 1982): and (c) 
parse failure is handled by the fitting procedure described here. 
\[n using the terms core grammar and periphery we are 
consciously echoing recent work in generative grammar, but we 
are applying the terms in a somewhat different way. Core 
grammar, in current linguistic theory, suggests the notion of a 
set of very general rules which define universal properties of 
human language and effectively set limits on the types of 
grammars that any particular language may have; periphery 
phenomena are those constructions which are peculiar to par- 
ticular languages and which require added rules beyond what 
the core grammar will provide (Lasnik and Freidin 1981 ) Our 
current work is not concerned with the meta-ruies of a Univer- 
sal Grammar. But we have found that a distinction between 
core and periphery is useful even within a grammar of a panic- 
ular language ~ in this case, English. 
This paper first reviews parsing in EPISTLE, and then 
describes the fitting procedure, followed by several examples 
of its application. Then the benefits of parse fitting and the 
results of using it in our system are discussed, followed by its 
relation to other work. 
93 
Parsing in EPISTLE 
EPISTLE's parser is written in the NLP programming 
language, which works with augmented phrase structure rules 
and with attribute-value records, which are manipulated by the 
rules. When NLP is used to parse natural language text, the 
records describe constituents, and the rules put these constitu- 
ents together to form ever larger constituent (or record) struc- 
tures. Records contain all the computational and linguistic 
information associated with words, with larger constituents, 
and with the parse formation. At this time our grammar is 
sentence-based; we do not, for instance, create record struc- 
tures to describe paragraphs. Details of the EPISTLE system 
and of its core grammar may be found in Miller et al., 1981, 
and Heidorn et al., 1982. 
A close examination of parse trees produced by the core 
grammar will often reveal branch attachments that are not 
quite right: for example, semantically incongruous preposition- 
al phrase attachments. In line with our pragmatic parsing 
philosophy, our core grammar is designed to produce unique 
approximate parses. (Recall that we currently have access 
only to syntactic and morphological information about constit- 
uents.) In the cases where semantic or pragmatic information 
is needed before a proper attachment can be made, rather than 
produce a confusion of multiple parses we force the grammar 
to try to assign a single parse. This is usually done by forcing 
some attachments to be made to the closest, or rightmost, 
available constituent. This strategy only rarely impedes the 
type of grammar-checking and style-checking that we are 
working on. And we feel that a single parse with a consistent 
attachment scheme will yield much more easily to later seman- 
tic processing than would a large number of different struc- 
tures. 
The rules of the core grammar (CG) produce single ap- 
proximate parses for the largest percentage of input text. The 
CG can always be improved and its coverage extended; work 
on improving the EPISTLE CG is continual. But the coverage 
of a core grammar will never reach 100%. Natural language is 
an organic symbol system; it does not submit to cast-iron 
control. For those strings that cannot be fully parsed by rules 
of the core grammar we use a heuristic best fit procedure that 
produces a reasonable parse structure. 
The Fitting Procedure 
The fitting procedure begins after the CG rules have been 
applied in a bottom-up, parallel fashion, but have failed to 
produce an S node that covers the string. At this point, as a 
by-product of bottom-up parsing, records are available for 
inspection that describe the various segments of the input 
string from many perspectives, according to the rules that have 
been applied. The term fitting has to do with selecting and 
fitting these pieces of the analysis together in a reasonable 
fashion. 
The algorithm proceeds in two main stages: first, a head 
constituent is chosen; next, remaining constituents are fitted in. 
In our current implementation, candidates for the head are 
tested preferentially as follows, from most to least desirable: 
(a) VPs with tense and subject; 
(b) VPs with tense but no subject: 
(c) segments other than VP: 
(d) untensed VPs. 
If more than one candidate is found in any category, the one 
preferred is the widest (covering most text). If there is a tie 
for widest, the leftmost of those is preferred. \[f there is a tie 
for leftmost, the one with the best value for the parse metric is 
chosen. If there is still a tie (a very unlikely case), an arbi- 
trary choice is made. (Note that we consider a VP to be any 
segment of text that has a verb as its head element.) 
The fitting process is complete if the head constituent 
covers the entire input string (as would be the case if the 
string contained just a noun phrase, for example, "Salutations 
and congratulations"). If the head constituent does not cover 
the entire string, remaining constituents are added on either 
side. with the following order of preference: 
(a) segments other than VP; 
(b) untensed VPs: 
(c) tensed VPs. 
As with the choice of head. the widest candidate is preferred 
at each step. The fit moves outward from the head. both 
leftward to the beginning of the string, and rightward to the 
end. until the entire input string has been fitted into a best 
approximate parse tree. The overall effect of the fitting proc- 
ess is to select the largest chunk of sentence-like material 
within a text string and consider it to be central, with left-over 
chunks of text attached in some reasonable manner. 
As a simple example, consider this text string which ap- 
peared in one of our EPfSTLE data base letters: 
"Example: 75 percent of $250.00 is $187.50." 
Because this string has a capitalized first word and a period at 
its end. it is submitted to the core grammar for consideration 
as a sentence. But it is not a sentence, and so the CG will fail 
to arrive at a completed parse. However. during processing. 
the CG will have assigned many structures to its many sub- 
strings. Looking for a head constituent among these struc- 
tures, the fitting procedure will first seek VPs with tense and 
subject. Several are present: "$250.00 is". "percent of 
$250.00 is", "$250.00 is $187.50". and so on. The widest and 
leftmost of these VP constituents is the one which covers the 
string "75 percent of $250.00 is $187.50", so it will be chosen 
as head. 
The fitting process then looks for additional constituents 
to the left, favoring ones other than VP. \[t finds first the 
colon, and then the word "Example" In this ~tring the only 
constituent following the head is the final period, which is duly 
added. The complete fitted parse is shown in Figure I. 
The form of parse tree used here shows the top-down 
structure of the string from left to right, with the terminal 
nodes being the last item on each line. At each level of the 
tree (in a vertical column), the head element of a constituent is 
marked with an asterisk. The other elements above and below 
are pre- and post-modifiers. The highest element of the trees 
shown here is FITTED, rather than the more usual SENT. (It 
is important to remember that these parse diagrams are only 
shorthand representations for the NLP record structures, which 
contain an abundance of information about the string proc- 
essed.) 
The tree of Figure I. which would be lost if we restricted 
ourselves to the precise rules of the core grammar, is now 
available for examination, for grammar and style checking, and 
ultimately for semantic interpretation, It can take its place tn 
the stream of continuous text and be analyzed for what it is 
a sentence fragment, interpretable only by reference to other 
sentences in context. 
9L. 
FITTEDI---NP ...... NOUN----"Example" 
\[ ------'! : It 
I ---VP" 
I 
I 
I 
I 
I 
I ------ll , It 
.... NPI ..... QUANT---NUM* ...... "75" 
I ..... NOUN*---"percent" 
I ..... PPl ..... PREP ...... "of" 
I ..... MONEY, ...... $250.00" 
.... VERBS-_.,,is,, 
.... NP ...... MONEY,--"$187.50" 
Figm'e 1. An example fitted parse tree. 
FITTED I ---NP" I .... N91 ..... AJ9 ..... ADJ* .... "Good" 
I I I ..... NOUN'---" luck" 
I I .... CONJ* - - -" and" 
I I .... NP I ..... AJP ..... ADJ* .... "good" 
I I ..... NOUN*--- "se 11 ing" 
FIil,,,'e 2. Fitted noun phrase (fragment). 
FITTED I ---VP* 
I 
I 
I 
I 
I 
I 
I 
I 
I 
I ------" t 11 
.... AVP I .... ADV" .... "S econdly" 
.... NP I ..... AJP ..... ADJ * .... "the" 
I ..... N9 ...... NOUN*---"Annual" 
l ..... NP ...... NOUN*---"Commiss ion" 
I ..... NP ...... NOUN s---''Statement'' 
I ..... NOUN'---"tota I" 
.... VERB .... "s hou id" 
.... VERB.---"be" 
.... NP ...... MONEY*--" $ I a, 682.61 " 
i ---AVP ..... ADV* .... "not" 
I ---NP ...... MONEY*--"$ I ~, 682.67" 
I ------" • ,r 
Fit, urn 3. Fitted sentence with ellipsis. 
Further Examples 
The fitted parse approach can help to deal with many 
difficult natural language problems, including fragments, diffi- 
cult cases of ellipsis, proliferation of rules to handle single 
phenomena, phenomena for which no rule seems adequate, and 
punctuation horrors. Each of these is discussed here with 
examples. 
Fragments. There are many of these in running text; they 
are frequently NPs, as in Figure 2. and include common greet- 
ings. farewells, and sentiments. (N.b., all examples in this 
paper are taken from the EPISTLE data base.) 
Difficult cases of ellipsis. In the sentence of Figure 3, what 
we really have at a semantic level is a conjunction of two 
propositions which, if generated directly, would read: "The 
Annual Commission Statement total should be $14,682.61; the 
Annual Commission Statement total should not be 
S\]4.682.67." Deletion processes operating on the second 
proposition are lawful (deletion of identical elements), but 
massive. It would be unwise to write a core grammar rule that 
routinely allowed negadvized NPs to follow main clauses, 
because: 
(a) the proper analysis of this sentence would be obscured: 
some pieces -- namely, the inferred concepts -- are 
missing from the second part of the surface sentence; 
(b) the linguistic generalization would be lost: any two 
conjoined propositions can undergo deletion of identical 
(recoverable) elements. 
A fitted parse such as Figure 3 allows us to inspect the main 
clause for syntactic and stylistic deviances, and at the same 
time makes clear the breaking point between the two propost- 
tions and opens the door for a later semantic processing of the 
elided elements. 
Proliferation of rules to handle single phenomena. There 
are some English constructions which, although they have a 
fairly simple and unitary form, do not hold anything like a 
unitary ordering relation within clause boundaries. The voca- 
tive is one of these: 
(a) Bit/. I've been asked to clarify the enclosed letter. 
95 
F I TTE D I - - -NP ...... NOUN * --- "B i i i" 
---- '* t " 
---VP* I .... NP ...... PRON'---" I" 
i .... VERB .... " ' ve" 
I .... VERB .... "been" 
I .... VERB,---"asked" 
I .... INFCL i --INFTO---" to" 
I --VERB* ..... clarify" 
I --NP I ..... AJP ..... ADJ" .... "the" 
I ..... AJP ..... VERB'---" enclosed" 
I ..... NOUN'---" letter" 
Figure 4. Fitted sentence with initial vocative. 
FITTED ---NP I ..... AJP ..... ADJ* .... "Good" 
i ..... NOUN*---" luck" 
---PP l ..... PREP .... "to" 
I ..... NP ...... PRON*---"you" 
I ..... CONJ*---" and" 
I ..... NP ...... PRON*--- "yours" 
---CONJ .... "and" 
---VP* \[ .... NP ...... PRON*---" I" 
l .... VERB*---"wish" 
l .... NP ...... PRON*--- "you" 
\[ .... NP ..... AJP ..... ADJ* .... "the" 
l ..... ADV ..... "VERY" 
l ..... ADJ* .... "best" 
i .... PP ..... PREP .... "in" 
..... AJP ..... ADJ* .... "your" 
..... AJP ..... ADJ* .... "future" 
..... NOUN,---" e f forts" 
Figure 5. Fitted conjunction of noun phrase with clause. 
(b) I've been asked. BilL to clarify the enclosed letter. 
(c) I've been asked to clarify the enclosed letter. Bill. 
\[n longer sentences there would be even more possible places 
to insert the vocative, of course. 
Rules could be written that would explicitly allow the 
placement of a proper name. surrounded by commas, at differ- 
ent positions in the sentence ~ a different rule for each posi- 
tion. But this solution Lacks elegance, makes a simple phenom- 
enon seem complicated, and always runs the risk of overlook- 
mg yet one more position where some other writer might insert 
a vocative. The parse fitting procedure provides an alternative 
that preserves the integrity of the main clause and adds the 
vocative at a break in the structure, which is where it belongs. 
as shown in Figure 4. Other similar phenomena, such as par- 
entheticaI expressions, can be handled in this same fashion. 
Phenomena for which no rule seems adequate. The sen- 
tence "Good luck to you and yours and l wish you the very 
best in your future efforts." is. on the face of it. a conjunction 
of a noun phrase (or NP plus PP) with a finite verb phrase. 
Such constructions are not usually considered to he fully gram- 
matical, and a core grammar which contained a rule describing 
this construction ought probably to be called a faulty grammar. 
Nevertheless, ordinary English correspondence abounds with 
strings of this sort. and readers have no difficulty construing 
them. The fitted parse for this sentence in Figure 5 presents 
the finite clause as its head and adds the remaining constitu- 
ents in a reasonable fashion. From this structure later seman- 
tic processing could infer that "Good luck to you and yours" 
really means "1 express/send/wish good luck to you and 
yours" -- a special case of formalized, ritualized ellipsis. 
Punctuation horrors. In any large sample of natural lan- 
guage text, there will be many irregularities of punctuation 
which, although perfectly understandable to readers, can com- 
pletely disable an explicit computational grammar. In business 
text these difficulties are frequent. Some can he caught and 
corrected by punctuation checkers and balancers. But others 
cannot, sometimes because, for all their trickiness, they ~tre not 
really wrong. Yet few grammarians would care to dignify, by 
describing it with rules of the core grammar, a text string like: 
"Options: Al-(Transmitter Clocked by Dataset) 
B3-(without the 605 Recall Unit) CS-(with ABC 
Ring Indicator) D8-twithout Auto Answer) EI0- 
(Auto Ring Selective)." 
Our parse fitting procedure handles this example by building a 
string of NPs separated with punctuation marks, as shown in 
Figure 6. This solution at least enables us to get a handle on 
the contents of the string. 
96 
FITTED I ---NP ...... NOUN* --- "Opt ions" 
I ------" : " 
I ---NP ...... NOUN*--- "AI " 
I ------ "-- 1) 
I ---" (" 
I ---NP I ..... NP ...... NOUN*---" Transmi tier" 
I I ..... NOUNe---"Clocked" 
I ---PP I ..... PREP ..... 'by" 
I I ..... NOUN*---"Dataset" 
I ---" ) " 
I ---NP ...... NOUNS---"BY' 
I ------ )) " ') 
I ---PP* I .... " (" 
I .... PREP .... "without" 
I .... AJP ..... ADJ* .... "the" 
I .... QUANT- - -NUM • .... "6 0 5" 
I .... NP ...... NOUN*---"Recal i" 
I .... NOUN*---"Unit" 
I .... ") " 
: ---NP ...... NOUN*--- "C5" 
___,I_,! 
---PP I ..... " (" 
I ..... PREP ...... with" 
I ..... NP ...... NOUN*--- "ABC" 
I ..... NP ...... NOUN s--- "Ring'' 
I ..... NOUN*---" Indicator" 
I ..... ") " 
---NP ...... NOUN'--- "D8" ___,,_,, 
---PP I ..... " (" 
I ..... PREP .... "w£ thou," 
I ..... NP ...... NOUN*--- "AUTO" 
I ..... NOUN e ..... Answer" 
I ..... ") " 
---NP ...... NOUN*---"E 10" ___,t . 
---NP I ..... " (" 
I ..... NP ...... NOUN*---"Auto" 
I ..... NP ...... NOUN*--- "Ring'' 
I ..... NOUN*---"Selective" 
I ..... ") " 
------". " 
~re a. Fitted list. 
Benefits 
There are two main benefits to be gained from using the 
fitted parse approach. First, it allows for syntactic processing 
-- for our purposes, grammar and style checking -- to proceed 
tn the absence of a perfect parse. Second, it provides a prom- 
ising structure to submit to later semantic processing routines. 
And parenthetically, a fitted parse diagram is a great aid to 
rule debugging. The place where the first break occurs be- 
tween the head constituent and its pre- or post-modifiers usu- 
ally indicates fairly precisely where the core grammar failed. 
It should be emphasized that a fitting procedure cannot be 
used as a substitute for explicit rules, and that it in no way 
lessens the importance of the core grammar. There is a tight 
interaction between the two components. The success of the 
fitted parse depends on the accuracy and completeness of the 
core rules; a fit is only as good as its grammar. 
Results 
In December of 1981. the EPISTLE grammar, which at 
that time consisted of about 250 grammar rules and did not 
include the fitted parsing technique, was run on the data base 
of ?.254 sentences from business letters of various types, The 
input corpus was very raw: it had not been edited for spelling 
or other typing errors, nor had it been manipulated in any way 
that might have made parsing easier. 
At that time the system failed to parse 832. or 36%, of 
the input sentences. (It gave single parses for 41°%. double 
parses for lit, , and 3 or more parses for 12°'o.) Then we 
added the fitting procedure and also worked to improve the 
core grammar. 
Concentrating only on those 832 sentences which in De- 
cember failed to parse, we ran the grammar again in July, 
1982, on a subset of 163 of them. This time the number of 
core grammar rules was 300. Where originally the CG could 
parse none of these 163 sentences, this time it yielded parses 
(mostly single or double) for 109 of them. The remaining 54 
were handled by the fitting procedure. 
Close analysis of the 54 fitted parses revealed that 14 of 
these sentences bypass the core grammar simply because of 
missing dictionary information: for example, the CG contains 
a rule to parse ditransitive VPs (indirect object-taking VPs 
.97 
with verbs like "give" or "send"), but that rule will not apply 
if the verb is not marked as ditransitive. The EPISTLE dic- 
tionary will eventually have all ditransitive verbs marked prop- 
erly, but right now it does not. 
Removing those 14 sentences from consideration, we are 
left with a residue of 40 strings, or about 25% of the 163 
sentences, which we expect always to handle by means of the 
fitted parse. These strings include all of the problem types 
mentioned above (fragments, ellipsis, etc.), and the fitted 
parses produced were adequate for our purposes. It is not yet 
clear how this 25% might extrapolate to business text at large, 
but it seems safe to say that there will always be a significant 
percentage of natural business correspondence which we can- 
not expect to parse with the core grammar, but which responds 
nicely to peripheral processing techniques like those of the 
fitted parse. (A more recent run of the entire data base result- 
ed m 27% fitted parses.) 
Related Work 
Although we know of no approach quite like the one 
described here, other related work has been done. Most of 
this work suggests that unparsable or ill-formed input should 
be handled by relaxation techniques, i.e., by relaxing restric- 
tions in the grammar rules in some principled way. This is 
undoubtedly a useful strategy -- one which EPISTLE makes 
use of, in fact, in its rules for detecting grammatical errors 
(Heidorn et al. 1982). However. it is questionable whether 
such a strategy can ultimately succeed in the face of the over- 
whelming (for all practical purposes, infinite) variety of ill- 
formedness with which we are faced when We set out to parse 
truly unrestricted natural language input. If all ili-formedness 
is rule-based (Weischedel and Sondheimer 1981, p. 3), it can 
only be by some very loose definition of the term rule, such as 
that which might apply to the fitting algorithm described here. 
Thus Weischedel and Black, 1980, suggest three tech- 
niques for responding intelligently to unparsable inputs: 
(al using presuppositions to determine user assumptions; 
this course is not available to a syntactic grammar like 
EPISTLE's; 
Ibl using relaxation techniques; 
(cJ supplying the user with information about the point 
where the parse blocked; this would require an interac- 
tive environment, which would not be possible for every 
type of natural language processing application. 
Kwasny and Sondheimer. 1981. are strongproponents of 
relaxation techniques, which they use to handle both cases of 
clearly ungrammatical structures, such as co-occurrence viola- 
r~ons like subject/verb disagreement, and cases of perfectly 
acceptable but difficult constructions (ellipsis and conjunc- 
tion). 
Weischedel and Sondheimer. 1982. describe an improved 
ellipsis processor. No longer is ellipsis handled with relaxation 
techniques, but by predicting transformatwns of previous pars- 
ing paths which would allow for the matching of fragments 
with plausible contexts. This plan would be appropriate as a 
next step after the fitted parse, but it does not guarantee a 
parse for all elided inputs. 
Hayes and Mouradian, 1981. also use the relaxation me- 
thod. They achieve flexibility in their parser by relaxing con- 
sistency constraints (grammatical restrictions, like Kwasny and 
Sondheimer's co-occurrence violations) and also by relaxing 
ordering constraints. However. they are working with a 
restricted-domain semantic system and their approach, as they 
admit, "does not embody a solution for flexible parsing of 
natural language in general" (p. 236). 
The work of WilLS is heavily semantic and therefore quite 
different from EPISTLE, but his general philosophy meshes 
nicely with the philosophy of the fitted parse: "It is proper to 
prefer the normal...but it would be absurd...not to accept the 
abnormal if it is described" (WilLs 1975, p. 267). WilLS" 
approach to machine translation which involves doing some 
amount of the translation on a phrase-by-phrase basis is rele- 
vant here. too, With fitted parsing, it might be possible to get 
usable translations for strings that cannot be completely parsed 
with the core grammar by translating each phrase of the fitted 
parse separately. 
Acknowledgements 
We would like to thank Lance Miller for his interest and 
encouragement in this work, and Roy Byrd, Martin Chodorow, 
Robert Granville and John Sown for their comments on an 
earlier version of this paper. 

References 

Hayes, P.J. and G.V. Mouradian. 1981. "Flexible Parsing" in 
American Journal Computational Linguistics. 7.4. 232-242. 

Heidorn, G.E. 1972. "Natural Language Inputs to a Simula- 
tion Programming System." Technical Report NPS- 
55HD72101A. Monterey, Calf Naval Postgraduate School. 

Heidorn, G.E. 1982. "Experience with an Easily Computed 
Metric for Ranking Alternative Parses" in Proc. 20th Annual Meeting of the ACL. Toronto, Canada, 82-84. 

Heidorn. G.E., K. Jensen. L.A. Miller. R.J. Byrd, and M.S. 
Chodorow. 1982. "The EPISTLE Text-Critiquing System" in IBM gys, J. 21.3. 305-326. 

Kwasny, S.C. and N.K. Sondheimer. 1981. "Relaxation Techniques for Parsing iU-Formed Input" in American Journal Computational Linguistics. 
7.2. 99-108 

Lasnik, H. and R. Freidin. 1981. "Core Grammar, Case 
Theory. and Markedness" in Proc. 1979 GLOW Conf. 
Pisa, Italy. 

Miller. L.A., G.E. Heidorn and K. Jensen. 1981. "Text-Critiquing with the EPISTLE System: An Authors's Aid to 
Better Syntax" in AFIPS Con/'. Proc.. Vol. 50. Arlington. 
Va., 649-655. 

Weischedel. R.M. and J.E. Black. 1980. "Responding Intelligently to Unparsable Inputs" in American Journal Computational Linguistics. 6.2. 
97-109. 

Weischedel, R.M. and N.K. Sondheimer. 1981. "A Framework for Processing Ill-Formed Input." Research Report. 
Univ. of Delaware. 

Weischedel, R.M. and N.K. Sondheimer. 1982. "An Improved Heuristic for Ellipsis Processing" in Proc. 20th Annual Meeting of the ACL. Toronto, Canada, 85-88. 

Wilks. Yorick, 1975, "An Intelligent Analyzer and Understander of English" in Comm. ACM 18.5. 264-274. 
