CONSTRAINTS ON NOUN PHRASE CONJUNCTION: A DOMAIN-INDEPENDENT 
MECEAZI~ 
Lynett • Hirechman 
Research and Development, Federal and Special Systems Group, 
Burroughs Corporation, Paoli, Pennsylvania, USA 
Coordinate conjunction is any important device for 
expressing complex information. To provide a habitable langua- 
ge processing system, most applications require some treatment 
of conjunction. When a rich conjunction mechanism is introduc- 
ed to account for the wide variety of conJotnings found in 
continuous text (as opposed to queries), it also becomes cruc- 
ial to restrict the parses obtained for the o0nJoinings, since 
con~nction is a major source of ambiguity in parsing. For 
example, the sequence of syntactic classes: 
NO~N PREPOSITION NOUN and NOUN 
has two possible pareeez 
NOUN I~%EPOSITION (NOUN and NOUN) 
swelling of (hands and feet) 
I NOUN PREPOSITION NOUN) and NOUN swelling of hands) and fever 
The choice of the correct parse depends on applyin~ 
domain-specific constrs/nts to determine which words "go to- 
gether" best. The concept of oonJunotional compatibility de- 
pends on semanti~ psxalleliam between the conjoined elements! 
thus, in the first example, hands and feet are more semantic- 
ally parallel than swellln~ and feet. Although it is possible 
to write special rules to define allowable con~oln~ngs for a 
partlaular domain of application, this would mean that the 
- 129 - 
system would not be portable: the conjunction constraints 
would have to be rewritten for each new domain. It is, how- 
ever, possible to formulate a general domain-independent 
principle to capture conjunction constraints. This paper will 
describe such a principle, implemented as "conjunction re- 
strictions" within the general treatment of conjunction in 
the natural language processing system of the Linguistic 
String Project parser. The illustrative examples are drawn 
from actual text occurrences in hospital discharge sunnnaries, 
processed using the mechanisms described below, as well as 
from some building code specifications, also processed with 
the LSP system. 
The first rule of conJunctional compatibility is simply: 
Two conjoined nouns must belong to the same (domain- 
specific) semantic class, 
This rule will produce the correct parse for the two previous 
examples, swellin~ of hands and feet and swelli~ of hands 
and fever. Hands and fee_._~t are both BODY-PART words in the 
first example, while swellin~ is not; in the second example, 
swellin~ and feve.._...rr are both SIGN.SYMPTOM words. 
In order to determine conJunctional compatibility, the 
noun conjuncts are compared pairwise! each conjunct is ini- 
tially assigned a list consisting of its semantic class(es). 
(Most words belong to Just one semantic class! however, cer- 
tain lexically ambiguous wo~ds belong to two or more classes, 
as illustrated below.) The intersection of the two lists of 
semantic classes is then computed. If the intersection is not 
empty, then the conjoining is allowed, and the intersection 
replaces the original list of classes associated with each 
conjunct. This serves as a record of a~v disambignation that 
occurs as a result of computing oonJunctionsl compatibility. 
For example, discharge is lexically ambiguous in medical 
narrative: it belongs to the ~DICAL-ACT class, as in: 
- 130 - 
hospital e~m'Leslen end dlso~ge 
INITIAL LISTS (INST) (MEDICALvACT) (MEDICALvACT,SIGN- s~su~oM) 
AFTER CONJ. (INST) (KEDICAL-ACT) (MEDICAL-ACT) 
whereas it has the SIGN-SYMPTOM reading in the phrase: 
no bleeding or discharge 
I~TIAL LISTS (SI~-SYM~OM) (~DICA'-ACToSION-S~U~O~) 
AW~ CONJ. (SI~W-S~U~OM) (SIGN-S~OM) 
Thus oomputin~ conjunction compatibility also provides a 
record of the correct reading when a conjunct is lexical!y 
ambiguous. 
The conJuBction rule does not account for the fact that 
certain noun phrases have a semantic class different fr.-. 
that of their head noun. Consider the following phrase from 
a building code text: 
bulldin~s and portions of buildings 
BLDG PART BLDG 
In this case, the head noun~ portion is "transparent" to its 
prepositional modifier, of buildln~se That is, the phrase 
portion of buildln~s really has a distribution characteristic 
of the word buildlnK. If we could disregard the word portions 
of and look instead at the noun in the prepositional phrase, 
namely buildin~ with class BLDG, then the conjunction rule 
would apply correctly. We call this type of construction the 
"computed attribute" oonstructlon, because, in its most gene- 
ral form, the semantic class for an entire phrase is computed 
from the interaction of the semantic class of the head noun 
and the olass(es) of its modifiers. 
The computed attribute construction turns out to be 
characteristic of a set of English semantic classes common to 
ma~7 domains. These classes include the classes AMOUNT, 
PERIOD, BEGINNING, ENDING, FREQUENCY, etc. To handle these 
- 131- 
constructions, an additional mechanism has been added as a 
restriction to the grammar. It operates on a word ~ belo~LUg 
to a transparent word class modified by an appropriate pre- 
nomiual noun or adjective modifier (e°g. buildin~ portion), 
or by an appropriate prepositional phrase modifier (e.g,, 
portions of ~ldin~s). It assigns to the phrase as a whole 
the class of the modifier and binds the modifier to the head 
noun, so that the modifier does not distribute over the other 
con~oined noun(s)° This produces only the ps.Tse 
buildings and (portions of buildings) 
.BLDG P~RT BLDG 
BLDG 4= COMPUTED ATTRIBUTE 
"and eliminates the parse (buildings and portions) of buildings, 
The computer attribute is recorded as a special COMPUT- 
ED-ATTRIBUTE list on the head noun; it lists the semantic 
olass(es) associated with the entire ~ The COMPUTED- 
-ATTRIBUTE llst is assigned before conJunetional compatibil- 
ity is checked, whloh allows the conjunction meahanism to 
make use Of a COMPUTED-ATTRIBUTE, if one is present° On the 
basis of a computed attribute and con~unctlon compatibility, 
the following sentence received exactly one analysis: 
There was no cyanosis and no history of prior seizures (SIaW-S~PTOM) (PERIOD) (SIOS-S~- 
\ / TOK) 
COI~UTED-ATTRIBUTE -> (SIGN.-SYMI~TO~) 
A preliminary test on a 22-sentence p~ph from a 
hospital discharge s~ was run to compare the. parses 
obtained with and without the above con~unction and computed 
attribute mechanisms. Five sentences in the paragraph contein- 
ed conjoined noun phrases. These mechanisms reduced the total 
number of parses obtained for the five sentences almost in 
half: 8 parses total with the conjunction and computed attri- 
bute mechanisms compared to 14 parses total without theme 
- 132 - 
The com~unction and computed attribute mechanisms do not re- 
solve all of the ambiguities of oomJoined noun phrases. In 
pe~tieular, they do not always resolve the issue of distribut- 
ion of right and left modifiers. However, they substantially 
reduce the number of incorrect parses and they require no 
modificstion for application to new domains. 
- 133 - 
