Is there content in empty heads?* 
Louise Guthrie, Brian M. Slator,** Yorick Wilks, and Rebecca Bruce 
Computing Research Laboratory 
Box 30001 
New Mexico State University 
Las Cruces, NM 88003-0001 
ABSTRACT 
We describe a technique for automatically constructing a taxonomy of word senses from a 
machine readable dictionary. Previous taxonomies developed from dictionaries have two pro- 
perties in common. First, they are based on a somewhat loosely defined notion of the IS-A 
relation. Second, they require human intervention to identify the sense of the genus term being 
used. We believe that for taxonomies of this type to serve a useful role in subsequent natural 
language proce,sing tasks, the taxonomy must be based on a consistent use of the IS-A relation 
which allows inheritance and transitivity. We show that hierarchies of this type can be 
automatical!y constructed, by using the semantic category codes and the subject codes of the 
Longman Dictionary of Contemporary English (LDOCE) to disambiguate the genus terms in 
noun definitkms. In addition, we discuss how certain genus terms give rise to other semantic 
relations between definitions. 
1. Introduction 
In order to extract meaning from text, we must 
at least disambiguate the words used in the text. The 
normal sceaario is to locate the disambiguated words 
in some knowledge base, which gives additional 
infc, m~,ation about the words and their properties, so 
that some of the higher-minded tasks of natural 
i;mgt.,age processing (NLP) can be accomplished: 
tasks iike determining speech acts, identifying topic 
a~_d focus shifts for discourse analysis, and drawing 
inferences. The work described here is a step reward 
developing an initiN knowledge base for NLP by 
automatmally transforming information found in 
machine readable dictionaries into a data base suit- 
able for a variety of NLP applications. 
The overall scheme of the work at CRL on 
Inachine readable dictionaries is described in Wilks et 
al. (1988, 1989, 1990). As part of that work, Slator 
(1988a, 1988b; Slator and Wilks, 1987, 1990) 
developed a program called Lexicon Provider which 
creates frames from the dictionary definitions of word 
senses provided by the Longman's Dictionary of 
Contemporary English (LDOCE). Each frame is con- 
** Prc~cnt address: Department of Computer Science, North 
Dakota State University, Fargo, ND 58105 
nected via an IS-A link to some other word (spelling 
form) in the dictionary. Our goal in this paper is to 
refine that work so as to connect each frame to 
another word sense in the dictionary. This insures 
that properties can be consistently inherited in this 
graph structure (since A IS-A B allows A to inherit 
properties of B). We can think of this task as build- 
ing a taxonomy of the word senses in LDOCE. 
This paper presents our techniques for automat- 
ing this task for noun definitions, using the special 
c~×led information found in the machine readable ver- 
sion of LDOCE. We also present ways to extract 
other semantic relations automatically as part of the 
process. 
2. Background 
Dictionary definitions of nouns are normally 
written in such a way that one can identify a "genus 
term" for the headword (the word being defined) via 
an IS-A relation. The information following the 
genus term, the differentia, serves to differentiate the 
* ~Pnis research was supported by the New Mexico State 
University Computing Research Laboratory through NSF 
Grant No. 1RI-8811108 -- Grateful acknowledgement is ac- 
corded to all the members of file CRL Natural Language 
Group for their comments and suggestions. 
138 -1o 
headword from other headwords with the same 
genus. For example, (from LDOCE): 
knife - a blade fixed in a handle, used for cut- 
ting as a tool or weapon. 
Here "blade" is the genus term of the headword 
"knife" and "fixed in a handle, used for cutting as a 
tool or weapon" yields differentia. In other words, a 
"knife" IS-A "blade" (genus) distinguished from other 
blades by the features of its differentia. In order to 
create a taxonomy of word senses, this genus term 
must be identified and also sense-tagged (in this case, 
by ruling out blade of grass, propeller blade, and an 
amusing tcllow). 
Previous research on constructing taxonomies 
from machine readable dictionaries, i.e. Amsler & 
White (1979) and, to some extent, Chodorow et. al. 
(1985), has relied on a good deal of human interven- 
tion whenever the taxonomy is composed of word 
senses rather than spelling forms. Nakamura 8: 
Nagao (1988) automatically constructed a utxonomy, 
but did not distinguish the senses of nouns and hence 
cannot allow inheritance of properties along the links 
of the implied network created by the taxonomy. 
Because of the semantic category markings in 
LDOCE, we have been able to develop heuristic pro- 
cedures (described in section 4), that, to a great 
extent, automate the task of developing a hierarchy of 
word senses. 
Constructing t,%xonomies from tt, e genus terms 
of definitions forces one to take a stand on how to 
treat a large class of noun definitions which are not as 
"standard" as the definition given above for knife. 
The characteristic property of these definitions is that 
the head of the first noun phrase (the usual place to 
find a genus term) seems vacuous, and another easily 
identifiable noun in the definition gives information 
about the headword. Nakamura & Nagao (1988), 
identify these non-sumdard definitions syntactically 
as: 
{det.} {adj.}* <Function Noun> of <Key Noun> {adj. phrasc}* 
For example, the following definitions have the pro- 
perty that the head of the noun phrase following the 
"of" is more semantically relevant to the headword 
than the head of the first noun phrase. 
arum (LDOCE) - a tall, white type of Lily 
cyclamate (LDOCE) - any of various man- 
made sweeteners ... 
deuterium (Meniam-Webster Pocket Diction- 
,'try) - a form of hydrogen that is twice 
the mass of ordinary hydrogen 
academic (LDOCE) - a member of a college or 
university 
The form of this type of definition is predictable 
whenever certain words ,are used as the head of the 
tirst noun phrase. Amsler and White (1979) kept a 
list of these words, referring to them as partives and 
collectives. Nakamura & Nagao (1988) call them 
Function Nouns. Chodorow et al., (1985) refer to a 
subset of these as "empty heads". Since we diS- 
Agree with certain elements of these characteriza- 
tions, we will use the terminology "disturbed heads". 
The question at issue is: what to do with these cases? 
In the original work of Amsler and White 
(1979) with the Merriam Webster Pocket Dictionary 
(MPD, 1964), file disturbed head cases were handled 
by asking paid human "disambiguators" to sense-tag 
the head of the first noun phrase in the definition and 
also to sense-tag any other noun in the definition 
which "made a significant semantic contribution to an 
IS-A link" (Amsler and White, 1979: p. 55) with the 
headword being defined (i.e. for the deuterium 
definition above, "hydrogen" was sense tagged as 
well as "form"). The taxonomy actually containexl 
both a link from deuterium to "form" and a link 
from deuterium to "hydrogen", although the hydro- 
gen sense was marked in a special way to indicate it 
is not the syntactic head of the definition. In cases 
like the "hydrogen" example just given, the marked 
"semantic contributors" were never given ancestors, 
since the link often represented a more loosely 
defined relation than the strictly transitive "is a subset 
of' definition of IS-A, which ideally relates the head- 
word and its genus sense. This degenerate fo,zn of 
IS-A precludes inheritance in the network. It is 
included in the taxonomy in order to form links to 
words which may not be related in a strict IS-A 
sense, but which convey useful information about the 
word being defined. 
There have been various proposals over the 
years suggesting different specialized link types to be 
added to the taxonomy (besides the degenerate IS-A). 
Markowi~ et al., (1986) suggest HAS_MEMBER 
links be created in definitions which use the phrase 
"member of" (i.e. "college" HAS_MEMBER 
"academic" in the definition of academic above). 
Nakamura & Nagao (1988) identify 41 different func- 
tion nouns and replace the IS-A link in their taxon- 
omy with various other links in these cases (except in 
the "kind of", "type of", etc., definitions). Amster 
(1980) suggests the incorporation of an IS_PART_OF 
link in addition to the IS-A links in the earlier taxon- 
omy of Amsler & White (1979). 
Chodorow et ~d., (1985) automate the genus 
finding process for nouns and verbs in Webster's 
Seventh (W7, 1967). However, in their work, only 
the spelling form of the genus is identified automati- 
cally; the sense selections are made by humans. The 
disambiguation here is not to attach a sense number, 
but rather to perform a function termed "sprouting" 
-2- 139 
which interactively selects among all words which 
have a given word-sense as a genus. Their taxonomy 
contains only IS-A links, but they partially attack the 
"disturbed head" problem by identifying a small class 
of what they call "empty heads". The effect of their 
method is to skip over seemingly vacuous terms 
(located where a genus is usually expected), and treat 
the more semantically relevant term as the actual 
genus. 
3. Description of LDOCE and its limitations 
The Longman Dictionary of Contemporary 
English (LDOCE; Procter et at. 1978), is a full-sized 
dictionary designed for learners of English as a 
second language that contains 41,122 headword 
entries, defined in terms of 72,177 word senses, in 
machine-readable form (a type-setting tape). The 
book and tape versions of LDOCE both use a system 
of grammatical codes of about 110 syntactic 
categories which vary in generality from, for exam- 
ple, noun to noun/count to noun/count/Jbllowed-by- 
infinitive-with-TO. The machine readable version of 
LDOCE also contains "box" and "subject" codes that 
are not found in the book. The box codes use a set 
of primitives such as abstract, concrete, and animate, 
organized into a type hierarchy. This hierarchy of 
primitive types conforms to the classical notion of the 
IS-A relation as describing proper subsets. These 
primitives are used to assign type restrictions on 
nouns and adjectives, and type restrictions on the 
arguments of verbs. The subject codes are another set 
of terms organized into a hierarchy. This hierarchy 
consists of main headings such as engineering with 
subheadings like electrical. These terms are used to 
classify words by subject. For example, one sense of 
current is classified as geology-and-geography while 
another sense is marked eragineering/electrical, 
This paper's overall goal is to make implicit 
semantic information in the dictionary explicit. How- 
ever, we are not doing "psychology of lexicography": 
the test of our derived structures is not whether they 
match any conscious or unconscious inferences of 
lexicographers, but whether they improve subsequent 
natural language processing (e.g. machine transla- 
tion). Nor are we in any way concerned here with 
low-level issues of the syntax of dictionary entries, its 
expression on tapes or pages, or by what device the 
information enters the computer. It is of course a 
strong assumption that a fallible dictionary designed 
for human learners of a second language also impli- 
citly contains the information needed for successful 
natural language processing. We make this assump- 
tion consciously as an empirical hypothesis. Even 
though LDOCE has beneficial features, such as its 
restricted vocabulary for sense definition, we see no 
reason to believe at this stage that the taxonomic rela- 
tions we derive are in any way non-standard. 
4. Automatically finding genus senses 
A heuristic procedure that automatically finds 
disambiguated genus terms for nouns has been 
developed. The initial stage of this procedure is to 
automatically identify the genus term in the 
definition. The Lexicon Provider (Slator 1988a, 
1988b; Slator and Wilks, 1987, 1990) mentioned 
above has a parser which does this. The parser 
accepts LDOCE definitions as Lisp lists and produces 
phrase-structure trees. LDOCE sense definitions are 
typically one or more independent clauses composed 
of zero or more prepositional phrases, noun phrases, 
and/or relative clauses. The syntax of sense 
definitions is relatively uniform, and developing a 
grammar for the bulk of LDOCE has not proven to 
be an intractable problem. Chart parsing was 
selected for this system because of its utility as a 
grammar testing and development tool. The chart 
parser is driven by a context frec grammar of 100- 
plus rules and has a lexicon derived from the 2,219 
words in the LDOCE core vocabulary. The parser is 
left-comer, and bottom-up, with top-down filtering. 
The context-free grammar driving the chart parser is 
virtually unaugmented and, with certain minor excep- 
tions, no procedure associates constituents with what 
they modify. Hence, there is little or no motivation 
for assigning elaborate or competing syntactic struc- 
tures, since the choice of one over the other has no 
semantic consequence. Therefore, the trees are con- 
structed to be as "flat" as possible. The parser also 
has a "longest string" (fewest constituents) syntactic 
preference. The grammar is still being tuned, but the 
chart parser is already quite successful and works 
extremely well over a fairly wide range of examples 
from the language of content word definitions in 
LDOCE. Ninety-Five percent result in a parse tree 
for the entire definition text. Five percent of the ana- 
lyses fail at some point. In those cases where it fails 
the parser still returns a partial parse (of the leading 
constituents in the definition texO, and this is the 
most imporUmt part of a definition anyway. 
The second phase of this procedure is to find 
the correct sense of the genus term that has been 
identified by the parser. To do this, we have con- 
structed a program called the Genus Disambiguator, 
which takes as input the subject codes (pragmatic 
codes) and box codes (semantic category codes) of 
the headword, taken from the machine readable ver- 
sion of LDOCE, and the spelling form of the genus 
word which has been identified by the parser 
described above. The output is the correct sense of 
the genus word. 
The codes in LDOCE seem to support the 
thesis that the genus for a noun must be a noun, and 
that the semantic category of the genus word must be 
140 -3- 
the same as, or an ancestor of, the semantic category 
of the headword. The word ancestor refers to 
superordinate terms in the hierarchy of semantic 
codes defined by the Longman lexicographers. The 
strategy of the algorithm is: 
1. choose the genus sense whose semantic 
codes identically match with the head- 
word, if possible; 
2. if not, choose the sense whose semantic 
category is the closest ancestor to the 
semantic category of the headword; 
3. in the case of a tie, the subject codes are 
used to determine the winner; 
4. if subject codes cannot be used to break the 
tie, the first one of the tied senses which 
appears in the dictionary is chosen (since 
more frequently used senses are listed 
first in LDOCE), 
The lollowing examples illustrate the algo- 
rithm. The ordered pair following the headword con- 
sists of the box code and subject code as found in 
dictionary (the notation following that is the English 
gloss for these particular codes). Many definitions 
are not given a subject code in LDOCE mid a dash 
(--) is used here to indicate that. Consider the fol- 
lowing LDOCE definition. 
ambulance - (J:movable-solid, AUZV: Auto- 
motive /Vehicle-Types) .- motor vehicle 
for carrying sick or wounded people esp. 
to hospital 
The genus of ambulance is the word "vehicle", 
which is fl)und by the Lexicon Provider's parser; 
therefore the input to the Genus Disambiguator is: 
(ambulance J AUZV vehicle) 
The following are the LDOCE definitions for the 
noun senses of vehicle° 
vehlcN!°l - (J:movable-solid, TNVH: Transpor- 
tation /Vehicles) - something in or on 
which people or goods c,'m be carried 
from one place to another ... 
vehicle-2 (T:abstract,--) something by 
means of which something else can be 
passed on or spread: Television has 
become an important vehicle for spread- 
ing political ideas 
vehicle-3 - (T:abstract,--) - a means for show- 
ing off a person's abilities: The writer 
wrote this big part in his play simply as 
a vehicle for the famous actress 
In this case the Genus Disambiguator chooses the 
tirst sense of vehicle, because of the match between 
the "movable-solid" semantic codes, therefore the 
output is "vehicle-l". There are many cases, however, 
where a direct match is not found. Consider the fol- 
lowing LDOCE definition. 
dart ° (J:movable-solid,GA:Games) o a small 
sharpwpointed object to be thrown, shot, 
etc .... 
The word "object" is the genus of dart, making the 
input to the Genus Disambigalator 
(dart J GA object) 
The following are the LDOCE noun definitions for 
"object" 
object-1 - (S:movable-solid,--) - a thing 
object-2 o (l:human-and~solid,--) - something 
or someone that produces interest or 
other effect ... 
object-3 ~ (l:human-and-solid,--) - something 
or someone unusual or that causes 
laughter 
object-4 - (T:abstract,-~) - purpose; aim 
object-5 (T:abstract,LN:Linguistics-and- 
Grammar) - word(s) saying with whom 
or with what, a PREPOSITION ... 
In this example there is no direct match between the 
semantic codes of the headword, dart, and any of the 
senses of the genus, "object"; therefore the Genus 
Disambiguator must traverse up the type hierarchy, 
described in section 3, to find the closest ancestor of 
boxcode "J" (movable-solid) that is present in the 
definitions of the genus word. In this case, boxcode 
"S" (solid) is found one level above "J" and the out- 
put is "object-l". There are still other cases, how- 
ever, when more than one sense definition has seman- 
tic codes matching the codes of the headword. Con- 
sider the following LDOCE definition. 
flute - (J:movable-solid,MU:Music) - a pipelike 
wooden or metal musical instrument with 
finger holes, played by blowing across a 
hole in the side ... 
The genus of flute is the word "instrument"; there- 
fore, the input to the Genus Disambiguator is 
(flute J MU instrument) 
The following ,are the LDOCE definitions for instru- 
ment° 
instrumentol (J:movable-solid, HWZT: 
Hardware/Fools) - an object used to help 
in work: medical instruments 
instrument-2 - 0:movable-solid,MU:Music) - 
... an object which is played to give 
musical sounds (such as a piano, a horn, 
etc.) ... 
instrument-3 - (Z:unmarked,--) - someone or 
something which seems to be used by an 
outside force to cause something to hap- 
pen: an instrument of fate 
-4o 141 
In this case both the first and second senses of 
instrument are marked as "J", (movable-solid), 
which matches perfectly with the selection restriction 
for flute. However, the tie is broken by appeal to the 
subject code, Music, which selects the second sense 
of instrument as the genus of flute, and the output is 
"instrument-2". 
There are occasional failures, many of which 
appear to be due to unusual markings in LDOCE. 
For exmnple, the LDOCE definition for banana is: 
banana - (P:plant,PMZ5:Plant-Names) - any of 
several types of long curved tropical 
fruit, shaped like a thick finger, with a 
yellow skin and a soft, usu. sweet, inside 
... 
The genus of banana is the word "fruit", and the 
input to the Genus Disambiguator is 
(banana P PM fruit) 
The following are the LDOCE definitions for fruit. 
fruit-1 - (J:movable-solid,FO:Food) - an object 
that grows on a tree or bush, contains 
seeds, is used for food, but is not usu. 
eaten with meat or with salt 
fruit-2 - (S:solid,FO:Food) - these objects in 
general, esp. considered as food ... 
fruit-3 - (J:movable-solid,FO:Food) - a type of 
this object 
fruit-4 - (J:movable-solid,BO:Botany) a 
seed-containing part of any plant 
fruit-5 - (T:abstract,--) - a result, good or bad: 
His failure is the fruit of laziness 
fruit-6 - (M:male/human,--) - fellow (in the 
phr. old fruit) 
In this case, banana is marked as a "plant" but, for 
some reason, the likely candidates defined under fruit 
are all marked "solid" or "movable-solid". Since nei- 
ther solid nor movable-solid ,are ancestor to plant in 
the LDOCE type hierarchy they are all equally bad, 
from the point of view of the Genus Disambiguator, 
and the default is invoked, which is to choose the 
lowest numbered sense from among the competitors. 
Therefore the first sense is selected and the output is 
"fruit-l". This happens to be correct, but it is an 
unsatisfying resolution. 
In a piece of related work, Slator (1988a) has 
implemented a scheme in the Lexicon Provider which 
imposes deeper structure onto the LDOCE subject 
hierarchy (e.g. terms like Food, Botany, and Plant- 
Names in the "fruit" definitions above) relating these 
categories in a natural way, in order to discover 
important relationships between concepts within text. 
This manual restructuring simply observes that words 
classified under Botany have pragmatic connections 
to words classified as Plant-Names, as well as con- 
nections with other words classified under Science 
(connections not made by the LDOCE hierarchy as 
given), and that these connections are useful to 
exploit. 
The Lexicon Provider system relates these 
codes through a specially restructured hierarchy 
created for that purpose, making Communication, 
Economics, Entertainment, Household, Politics, 
Science, and Transportation the fundamental 
categories. Every word sense defined with a subject 
code therefore has a position in the new hierarchy, 
attached below the node for its subject code. Once 
this feature is implemented in the Genus Disambigua- 
tor, the subject code hierarchy can be used to resolve 
the "banana-fruit" case above in a somewhat more 
satisfactory way, by choosing sense 4 of fruit. 
5. Identifying other relationships automatically 
The identification of a satisfactory genus term 
and the construction of a taxonomy is not straightfor- 
ward in all cases. It is clear that the problems in this 
area are difficult, numerous, and can be seen to 
encompass a great variety of relationships. We 
believe that a thorough study of this shadowy area is 
necessary in order to make optimal use of the seman- 
tic information available in machine readable dic- 
tionaries. Although we do not have complete solu- 
tions, we have additional insights into the problem of 
extracting supplementary information from the "dis- 
turbed head" definitions. 
Chodorow et al. (1985) examined a 
phenomenon that they described as follows: 
"If the word found belongs to a small 
class of "empty heads" (words like one, 
any, kind, class, manner, family, race, 
group, complex, etc.) and is followed by 
of, then the string following of is repro- 
cessed in an effort to locate additional 
heads." (pg. 301). 
Although the empty head rule seems to be a 
reasonable one in certain situations, we have reserva- 
tions about its use. The empty head rule produces 
undesirable effects in an IS-A hierarchy for some of 
the collective words (that Chodorow et al. treat as 
empty): set, group, class etc. Our response to the 
empty head phenomenon is to process them in the 
same way, but limiting this processing to a much 
smaller set; that is, to those heads that are truly 
empty -- the set containing {one, any, kind, type}. 
Consider the LDOCE definition: 
canteen - (British English) a set of knives, 
forks and spoons, usu. for 6 or 12 peo- 
ple 
Since "set" is one of the empty heads for Chodorow 
et al., their procedure would create IS-A links to 
142 ..5- 
"knives", "forks" and "spoons", and this again would 
violate the inheritance properties that should be 
preserved via IS-A links. Our response to the collec- 
tive heads, {set, group, collection, class of, family 
of} (which we maintain are not truly empty, simply 
disturbed), is to form a taxonomic link to the correct 
sense of "set," "group," or "class" etc. and to form a 
HASMEMBER link to the noun or nouns which 
describe the elements of the collective (as found in 
the differentia of the headword definition). Further, 
we propose that definitions in which the genus term 
is plural be treated in the stone way as those which 
begin with "a set of''. 
In general, our view is that the disturbeA heads 
should be grouped in the sense of Nakamura & 
Nagao (1988), and that additional links (like 
HAS MEMBER, IS PART OF, etc.) should be 
created whenever they are appropriate. However, it is 
our position that IS-A links should also be created for 
every word sense given in the dictionary. Moreover, 
in order to maintain inheritance and transitivity in the 
IS-A network, a strict "subset of" definition of IS-A 
should be maintained. 
Unlike Nakamura & Nagao (1988), we propose 
that "member of'' definitions should not be grouped 
with the "set of", "group of" definitions. All but one 
"member of" definition in LDOCE uses ~'member of" 
to mean "person who is a member of". We recom- 
mend that in this case, a link be created from the 
headword to "person", and that the appropriate 
MEMBER-OF link is constructed. The exceptional 
case, where "member of`' does not refer to a person, 
is in the definition of feline : "a meml~er of the cat 
family." This case must be treated separately, since it 
is impossible to identify the correct sense of the word 
"member" here, given that all these senses, in 
LDOCE, are marked as referring to a human or a part 
of the human body. 
The difficulty of these many varieties of special 
cases (~td they are not so special, since there are 
hundreds of them in the dictionary), is that they call 
into question certain of the long held assumptions 
about the taxonomic structure of dictionaries. The 
conventional wisdom has always been that dictionary 
definitions contained a genus term (a term more gen- 
eral than the one being defined), and that this term 
could almost invariably be found in the first phrase of 
the definition text. Further, the exceptions to this 
convention, the "empty heads" like "one of" or "any 
of", have been viewed as being similarly well- 
behaved. Our investigations lead us to conch\]de that 
things are not so simple as they once appeared; and 
the question of what to do with these troublesome 
cases is far from resolved. 

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