i 
AN APPROACH TO NATURAL LANGUAGE IN THE SI-NETS PARADIGtl 
Amedeo Cappelll, Lorenzo lloretti 
Istituto di Llngulstlca Computazionale-CNR 
Via della Faggiola, 32 
56100 Plsa- Italy 
ABSTRACT 
Thls article deals with the interpretation 
of conceptual operations underlying the 
communicative use of natural language (NL) within 
the Structured Inheritance Network (Sl-Nets) 
paradigm. The operations are reduced to functions 
of a fo~al language, thus changing the level of 
abstraction of the operations to be performed on 
SI-Nets. In this sense, operations on SI-Nets are 
not merely isomorphic to single epistemologleal 
objects, but can be viewed as a simulation of 
processes on a different level, that pertaining to 
the conceptual system of NL. For this purpose, we 
have designed a version of KL-ONE which represents 
the eplstemologlcal level, while the new 
experimental language, KL-Conc, represents the 
conceptual level. KL-Conc would seem to be a more 
natural and intuitive way of interacting with 
SI-Nets. 
I GOALS 
The goal of our work is to interpret 
conceptual operations underlying the communicative 
use of natural language within the Structured 
Inheritance Networks (SI-Nets) paradigm. In other 
words, this means using eplstemological primitives 
such as Concepts, Roles and Structural 
Descriptions (Brachman, 1979), to represent these 
conceptual operations. 
On the one hand, epistemological formalism, 
which is explicit and clear, can clarify the 
behaviour of conceptual operations of NL. 
lSy the use of SI-Nets formalism as a means 
of description, a new perspective can be brought 
out, since this formalism makes it possible to 
represent objects as data types structured in a 
complex way instead of considering them as mezR 
atomic elements. This feature Is likely to change 
the nature of the operations to be carried out on 
objects thus leading us to deal with the 
complexity of many phenomena in a more adequate 
way. 
On the other hand, this can lead to an 
investigation of the relationships between the 
conceptual aspects of NL and the epistemological 
primitives, in order to discover how the latter 
are used by the previously taentioned operations. 
In fact, we attempt to find out whether an 
isomorphism exists between objects and operations 
of NL and those used by epistemology. 
According to Brachman (1979), five different 
approaches to the representational problem can be 
established: implementatlonal, logical, 
eplstemological, conceptual and linguistic. Each 
of them uses its own primitives so that the five 
levels can be interpreted as a hierarchy where 
each level involves different degrees of 
abstraction. 
By virtue of this interpretation, we have 
tried to extend epistemology in a conceptual 
perspective. Our current approach considers 
epistemology as a starting point, thus looking at 
the conceptual level as one of the possible target 
points. This goal can be achieved by changing the 
level of abstraction of the operations to be 
performed on SI-Nets. Consequently, operations on 
SI-Nets could assume a different aspect, that is 
to say they could be viewed not as merely 
isomorphic to single eplstemologieal objects but 
as a simulation of operations lylng on a different 
level, for instauce, that pertaining to the 
conceptual system of NL. This hypothesis can 
reduce SI-Nets to the level of an internal 
mechanism covering only abstract data 
representation, whose structure is not transparent 
to the user. In this case tile user interacts with 
the internal system by means of a separate 
external framework. 
In order to achieve this goal we have 
designed and i~Iplemented a language, KL-Hagma 
which represents our epistemologlcal level. We are 
now designing and Implementing an experimental 
language, KL-Co nc, which should cover the 
conceptual level and ~lhlch uses KL-itaglaa as one of 
its internal co,mpo.ents. 
The rest of the article will be devoted to a 
description of these two languages introducing 
considerations concerning their relevance to 
linguistic analysis and knowledge representation. 
We are confident that our approach can have 
interesting Ir,~plications for both these fields, 
122 
Since KL-Cone .functions can be used to describe 
linguistic entities in te~s of conceptual 
operations and may be viewed as a more natural way 
of interacting with SI-Nets. 
I I KL-MAGMA 
KL-Magma is a version of KL-ONE implemented 
In MACt~-Lisp (Aslrelll et al. 1975). 
It is a f~ language similar to the one 
described in Brach~nan (1979), Brac1~an et al. 
(1978), which also takes into account the versions 
given in Cappelll and Morettl (19S2) and Porta and 
Vlnchesl (1982). As in KL-One, ~\[L-Hagma formal 
objects are Concepts, Roles and Structural 
Descriptions. 
Concepts are descriptlonal structures 
providing an intenslonal representation of the 
domain, i. e. prototypes and individuals. 
Roles are descriptional structures 
representing parts of Concepts, i. e. properties 
of prototypes and individuals. 
Structural Descriptions are sets of 
relationships between Roles which give a whollstlc 
structure to Concepts. 
Objects are connected with one another via 
Cables and Wires, thus resllzing Structured 
Inheritance. 
In our current approach KL-Hsgma is mainly 
used as a declarative model of abstract data 
structures. It has no mechanism like the MSS 
Algorithm (Needs, 1981) or the KL-One Classifier 
(Sclmolze and Lipkis, 1983) which cover procedural 
aspects lying within epistemology, thus reaching 
valuable results in discovering new types of logic 
by deeply exploiting SI-Nets semantics. Instead, 
we have tried to discover types of procedurallty 
external to the eplstemological level and 
pertinent to the level we intend to represent. At 
any rate, we intend to govern epistemological 
processes by the external mechanism. In other 
words, this means assuming, for instance, the 
logic of subsumptlon , which is peculiar to 
epistemology, not as an autonomous deductive 
mechanism, but, instead, as a possible process 
controlled by the functions of the higher level 
language. 
III WIIAT TYPE OF CONCEPTUAL OPERATIONS ? 
The conceptual operations of NL we intend to 
interpret are, for instance, Indlvldustions of 
objects, evaluations of objects, evaluations of 
properties of objects, evaluations of 
configurations of objects and so on. 
Operations of this kind are trlggered by 
articles, adjectives, prepositional phrases, 
relative clauses and so on. These operations, 
already intuitively described in classical 
Linguistics, have been given more attention by 
investigations based on Logic. In the logic 
paradigm they can be viewed as classes, Sets, 
predicates etc.. 
In our opinion, the nature of these 
operations and, consequently, the description we 
intend to give of them, are not completely covered 
by logical analysis. Interesting results have been 
obtained by combining traditional logical systems 
with extensions of lambda calculi (~Jebber, 
1978;1981). However, the types of complex 
procedurality peculiar to the operations have no~ 
yet been given a precise description; that is to 
say, procedurality has not been reduced to 
definite sets of restricted and clear procedures. 
Let us now introduce an example. The 
Italian definite and indefinite articles (il, un) 
can be described as follows: 
a) indlvlduatlon of a specific object; 
b) indlviduation of any one object; 
c) reference to an abstract prototype. 
In terms of logical description a) and b) 
may correspond to the iota operator and the 
existential quantifier of Logic; c) is similar to 
the universal quantifier even if the notion of a 
prototype is different, since it has an 
intenslonal nature. 
However, we think that tlle three possible 
descriptions of Italian articles may include types 
of operations not covered by the use of the above 
mentioned logical operators. The article, like 
many other linguistic entities, integrates 
different kinds of operations which, at the same 
time, manipulate descriptions of prototypes and 
individuals, search into different kinds of 
memory, etc. 
Let us introduce a new example. The 
adjective is one of the more conlplex phenomena of 
NL which cannot be reduced to the notion of 
predicate since it triggers a set of reasoning 
processes, that is to say, the manipulation of 
parts of knowledge. 
The following NP: 
I. un bambino rosso 
may be interpreted as: a child has hair, 
hair has a color, the color can be red. This NP 
cannot be literally translated into English 
without adding more information; the appropriate 
translation is : a red-halted child. 
In terms of SI-Nots this process can be 
represented as shown in figure I, assuming that 
123 
every lexlcal item of the NP has its own 
intensional representation. 
Figure I 
However, the adjective does not specify all 
the steps of the reasoning process that it 
triggers, but only indicates, together with the 
name, the two extreme points of the chain leaving 
the intermediate undefined. The entire process, 
using generic knowledge as the reference point, is 
shown in Figure 2. 
Figure 2 
It would be oversimplifyiug, as stated above, 
to use the notion of predicate to interpret this 
complex process as well as the other possible 
interpretation of the adjective: the one 
corresponding to tile notion of "type of" aQ in the 
NP "a red color" (see fl~ure 3). 
Figure 3 
This type of phenomena can be investigated by 
deeply exploiting the structure and the semantics 
of SI-Nets. The structure of a role can be used 
as configuration of objects which are likely to be 
manipulated by complex processes not yet deeply 
investigated from any other viewpoint than the 
eplstemologlcal one. Once considered as a complex 
llnk, as it actually is, a role may be the locus 
where different processes can be triggered. It may 
be used simply to satisfy a structure of another 
role lying higher within the network or to trigger 
the complex processes we were talking about. The 
two behavlours mentioned exhibit different levels 
of abstraction; in the former case this means 
performing eplstemologlcal operations, while in 
the latter we simulate processes of a conceptual 
system used by NL. 
IV WHY A NEW LANGUAGE? 
The question now arises whether it is 
possible to reduce these types of operations to a 
set of functions of a formal language each of 
which covers a well defined process which 
corresponds to a well defined set of operations on 
SI-Nets - to a set of KL-Magma functions. 
The choice of a new language has many 
motivations: 
a) from the conceptual viewpoint, this means 
reducing operations to functions that are well 
defined from a semantic viet~polnt which lend 
clearness to tile process to be represented. 
a) from the epistemological viewpoint, it is 
reasonable to think that a language, such as 
KL-Magma, may be extended by another language thus 
achieving a higher degree of abstraction. 
c) a language is a uniform mechanism for the 
integration of interpreters of several symbolic 
processes. This integration is likely to bring out 
more clearly relevant phenomena of the process 
represented. 
V KL-CONC 
On the basis of the linguistic assumptions 
previously outlincd and using KL-Hagma as a 
language which handles SI-Nets, we are now 
designing and implementing an experimental, 
language, KL-ConL, whose functions try to simulate 
the conceptual operations previously described. 
A. KL-Conc: Internal Organization 
Before describing KL-Conc functions in 
detail, it is worth while discussing its internal 
organization. 
124 
In the framework of KL-oNE, a relevant 
distinction has been drawn between the 
Terminological Box (T-Box) and the Assertional Box 
(A-Box) (Brachman, 1981). The T-~ox maintains the 
detailed description of the objects while the 
A-Box contains the set of the assertions on the 
objects. The former corresponds to the ability of 
describing by the use of NPs, and the latter to 
that of constructing complex sentences. 
A discussion has arisen whether it Is 
possible to handle the two boxes, which correspond 
to two different areas of memory, using the same 
language. 
In KL-O~E, new functions have been added in 
order to glve it an assert~onal power (nexus, 
context) (Woods 1979). 
A recent extellsion of KL-O~IE (Brachmsn et 
al., 1983) has adopted the solution of cresting 
two distinct languages: one for the T-Box and the 
other for the A-Box. The fon~er Is a sort of 
KL-ONE viewed in a functional way while the latter 
Is a language based on First Order Predicate 
Logic. 
KL-~IACHA is only able to handle the T-Box 
and it has no assertional power. Instead, by 
KL-Cone we are trying to design a language which 
covers both terminological and assertional 
aspects, even if it is more biased towards 
assertlonallty. It is our intention to handle the 
T-Box mainly in an assertlonal way. 
In order to achieve thls goal we have 
introduced the distinction between Long Term 
Hemory (L~I) and ~orklng |~emory (~,I) which in part 
covers the traditional one between T-Box and 
A-Box. 
The LT~' is represented in EL-Magma data 
structures; this contains descriptional knowledge 
about generic and individual objects. 
The W~! contains the history of the objects 
organized in a structured way. This is the 
central component of our current hypothesis. The 
|;H contains the traces of contextual relationships 
between objects, as well as operations triggered 
on and by objects; it can also contain other 
symbolic systems. The task of the ~JM is mainly to 
hold hypotheses to be mapped onto the LT~! which 
requires the cooperation of several interpreters. 
The Introduction of a larger number of 
memory spaces increases the power of the language. 
For instance, a structured WU is likely to improve 
the number of s~nbolic systems interacting with 
one another. This makes It possible to insert into 
the language functions based on different 
processes. Taking for instance the history of the 
objects as a reference point, the objects 
themselves can be accessed according as they 
appear in the time flow. The function: 
<LAST arbitrary_name> 
returns the last object, created or manipulated, 
belonging to the class named by arbitrary n~ze. In 
other words, this allows the user to refer to 
objects using anaphorical references, that is to 
say using a s~nbolic system which is organized and 
represented in a different way from epistemology. 
By the WM we are trying to create the basic 
mechanism to handle these types of processes. 
B. KL-Conc: External Organization 
KL-Conc functions handle real world objects, 
so the user only needs to know a set of functions 
to be applied to objects. In this way, the 
structure of the Sl-?let which internally organizes 
the data, is hidden; the only t~ta which are 
transparent are objects, which may be individual 
or generic, together with syntactic rules for 
combining functions. These last are very flexible. 
Objects can be accessed using arbitrary names or 
by means of syntactic combinations which 
conceptually correspond to complex tests on the 
nature of objects, the configuration of objects 
etc.. Objects can be accessed according as they 
appear in the time flow. 
The user can use the same name both for 
generic and individual objects. This is made 
possible by means of an internal generator of 
names which, starting from the name of a generic 
object, provides any individual of that class with 
a different name. This feature covers the part of 
the naming system of NL which uses the same name 
for individuals and prototypes. This does not 
cover the use of proper names which has been taken 
in JARGON (Woods, 1979) as the only means for 
naming individuals, thus oversimplifying the real 
system used by NL (Mark, 1981). 
Objects can be accessed without the use of 
names, but by means of functions or combinations 
of functions in order to perfo~ complex tests on 
the nature of objects. This means referring to 
objects by testing properties or configurations. 
C. KL-Conc Functions 
KL-Conc has functions for creating, testing 
and retrieving objects. This is the list of tile 
functions so far designed: 
GEN 
NEUIND 
JUSTONE 
ANYO~:E 
SOME 
ALL 
LAST 
ADD PROPERTY 
ADD--CONFIGURATION OF PROPERTIES 
TEST PROPERTY 
TEST CONFICURATION OF PROPERTIES 
The semantics of some KL-Cone functions may 
now be described in order to clarify how they 
125 
realize our linguistic assumptions. The semantics 
is given in terms of operations on SI-Nets. 
As far as generic knowledge is concerned, 
the function: 
<GEL\] arbltrary_na~;,e> 
returns the generic concept named by 
arbitraryname. If the concept does not exist in 
the LTM a new generic concept is created. The new 
concept is then returned. This function works both 
as a predicate and as a creating function. It is 
worth noticing that in KL-Hogma there are two 
distinct functions, one for the predicate 
(<Generic__Concept._P anything>), and the other for 
creating (<Create Concept name type of concept>). 
The function 
<PEWI~:D arbitrary_name> 
creates a new individual concept and establishes 
it as an individuator of the generic concept named 
by arbitrary name; if the generic concept does not 
exist in t~e LT~I it is created. An internal 
generator provides tile newly created individual 
concept wlth o nnme. This function corresponds to 
the follo~llng set of KL-\[!agn.la functions: 
(Create_Concept X1Jndlvidual) 
((Hot (Generlc_Concept_P X) (Create Concept X" 
generic)) 
(Establish as Individuator X1 X) 
This is one of the most "declarative" 
functions since it creates a new individual 
concept without searching in the LTN. In other 
words, tile user must be conscious that the new 
object is added to the L lqq and it is different 
from all the other objects. A more psychological 
oriented behavlour would require to test in 
advance the nature of the new object in order to 
decide whether the object is similar to or 
coincides with an individual object already 
inserted into tile LTH. The salute problem has been 
overcome in KRYPTO.~: by means of tile swltcb 
TELL/ASK (Brachman et al., 19P3). 
The function 
<JUSTOr!E arbitrary name> 
verifies whether there exists a unique individual 
either named by arbltrary, name or defined by tests 
or combinations of tests according to KL-Conc 
syntax. In other words, this means verifying if 
the object is unique as to its name, or as to one 
of its properties etc. The KL-Conc expressions 
for the two meanings are, respectively: 
(JUSTO~E table) 
(JUSTONE (TESI~PROPERTY table red)) 
This function has a complex behavlour, 
since, intuitively, it must verify the unlqueness 
of an object and must return: i) the individual if 
unique; ll) the llst of individuals if more than 
one satisfies the conditions .given by assertions; 
Ill) NIL if no invlvldual exists satisfying the 
conditions (Carnap, 1947). The three answers have 
different meanln~s, since they imply different 
operations to be triggered on the memory spaces 
or, at any rate, they have different effects on 
the behavlour of functions where JUSTONE can be 
nested. 
The function: 
<TEST CONFIGURATION OF PROPERTIES 
~rbltrary_namel arbltrary_name2> 
verifies whether arbitrary name2 exists in the 
horizontal chain . of r~les starting from 
arbitrary_namel (see Figure 4) 
1 • • L°QO * 
Figure 4 
BY the function: 
<ADD_PROPERTY arbltrary__namel arbltrary_name2) 
we intend to add roles to concepts so that the 
user needs not have any specific kno~Jledge about 
the distinction between generic and instance roles 
or, seen from a different viewpoint, between 
properties of prototypes and properties of 
individuals. Taking NL as the reference point, we 
think that the above mentioned distinction is 
peculiar only to certain linguistic elements; in 
the case of operations on propertles, no 
distinction is made; it is the conceptual 
operations governing the operations on properties 
that control the correct application of the adding 
or testing properties. Consequently, the function 
ADD PROPERTY must be designed In order to make it 
pos~Ible to trigger the correct procedures 
depending on the type of objects which it is 
applied to. For this purpose, we intend to use a 
metarepresentation of KL-Hagma (Cappelli et al., 
1983) which, on detecting tile type of object, 
automatically apply the appropriate procedures. 
This implies a system which creates or tests 
knowledge structures interpreting its own syntax. 
Let's now briefly describe two possible 
behavlours of this function. 
Wtlen applied to individual concepts, thls 
creates a new instance role establishing it as a 
126 
satisfler of a higher generic role of the generic 
concept ancestor of the individual concept. If a 
possible generic role does not exist it is created 
without inserting any V/R in the generic role, 
since it could be a more general concept than the 
generic concept ancestor of the value of the newly 
created instance role. The structures created by 
this function are shown in figure 5 by dotted 
lines. 
"~0// I 
The functions described in this article 
represent only a subset of the operations which 
can be embodied in tile language. In this sense, 
the number of KL-Conc functions is likely to be 
increased in order to cover new processes. 
So far, we have designed the functions for 
those operations which exhibit the same behaviour 
whatever domain they are applied to, since they 
represent the "deep" behavlour of syntactic 
elements. It is to be emphasized that we have 
tried to reduce to the fomn of functions of a 
language, all the operations of NL which are 
domaln-lndependent and which represent aspects of 
the abstract syntactic ability of structuring 
knowledge facts (Cappelll etal., 1983; Cappelll 
and Moretti, 1983) 
Using KL-Conc it is possible to investigate 
how linguistic elements can be described in temns 
of conceptual operations. This is a further step 
towards the linguistic level. On reaching this 
level, the task will be to discover how the 
conceptual operations are embodied in linguistic 
forms. 
The previously mentioned Italian articles may 
be described as follows: 
Figure 5 
~en applied to generlc concepts, the 
function adds a new generic role, trying to link 
it with a higher generic role. If no generic role 
is found, a higher generic role is created without 
providing it with any information other than the 
one inferred from the structure of the newly 
created subrole. 
VI CONCLUSIOtIS 
Some conclusions may now be drawn both from 
a linguistic sad a knowledge representation 
viewpoint. 
From a linguistic viewpoint some relevant 
facts must be pointed out. 
First of all, the level of integration 
reached by the construction of a uniform language, 
can bring out more clearly the nature of many 
phenomena of \[;L, since it is possible to put 
together many processes which cooperatively 
contribute to the realization of a single 
phenomenon. This means looking at the complexity 
of HL with the aid of a powerful symbolic 
Instr~nent, capable of handling contemporaneously 
several aspects of that complexity, thus reaching 
a higher degree of adequacy. In designing 
KL-Conc, we aim to create a framework which can 
extend the possibility of investigating and 
representing these phenomena. 
(Definite Article lambda (x) 
(or (GE~ x) 
(Jus'rONE x))) 
(Indefinite Article lambda (x) 
(or (CE~ x) 
(ANYONE x) )) 
From a knowledge representation viewpoint 
KL-Conc would seela to be a means for interacting 
with SI-Nets in an intuitive way. The user is not 
required to have a specific knowledge of Sl-Nets 
fo~iallsm; he only needs to know a set of 
functions to be applied to objects. 
In this sense KL-Conc assumes a more natural 
aspect, thus overcoming the constraint of a 
structure-orlented language such as KL-Ha~zLa. 
This feature has been obtained by handling Sl-~ets 
in a more compact way. KL-Conc provides the user 
with a set of functions which are not isomorphic 
to single eplstemologlcal objects but which handle 
pieces of network starting from discontinuous 
information. 
This weakness, peculiar to NL, is made 
possible in KL-Conc by assuming the 
epistemologlcal level as a reference schema, 
instead of a reductlonlst for,uallsm. This means 
introducing mechanisms for relaxing the rules of 
KL-Ha~sa. In this way KL-Conc can be seen as a 
"constructive" system (in the sense of Korner 
1970) which manipulates its "'factual" system 
(KL-Magma) in an intultlonistic way. 
Finally, KL-Conc suggests a different way of 
exploiting spreading activation mechanisms 
(Quilllan, 1968) using several symbollc systems 
127 
organized by the ~II instead of considering the,n as 
algorithmic devices internal to SI-Nets (Woods, 
198~: 
VII P, EFERENCES 
Asirelll P., C. Lami, C. l:ontangero, G. Pacini, H. 
Slmi, F. Turin|, ":L~CHA-Lisp Reference Hanual", 
N.T. C75-13, I.E.I CNR, Pisa , 1975. 
Brachman R., E. Clccarelll, ~. Greenfeld, H. 
Yonke, "KLONE P.eference ~4anual", Report n. 
3848, Bolt Beranek and Ne~.~:mn Inc., Cambridge 
(Hass.), July 1978. 
Brachman R. J., "On the Eplstet~3ologlcal Status of 
Semantic Networks", in N. Findler (ed.), 
Associative Networks: llepresentatlon and Use of 
Knowledge by Computers, New York: Academic 
Press, 1979. 
Brachman R.J. and If. Levesque, "Assertions in 
KL-ONE", in Schmolze J. and Brachman R. (Eds.), 
Proceedings of the 1981 I'L-Or:E Workshop, Report 
n.4842, Bolt F, eranek and Hewman Inc.,Cambrldge 
(Hass.), June 19R2. 
Braci~aan R.J., R. E. Fikes, 11. J. Levesque, 
"KRYPTON: A Functlonal Approach to Knowledge 
Representation", Fairchild Technical Report 
n.639, FLAIR Technical Report n.16, Pale Alto 
(Ca.), May 1983. 
Cappelli A. and L. Horettl, Introduzlone al 
KL-ONE, ILC-L~LP-1982-l, I.L.C. CUR, Pisa, 1982. 
Cappelll A. , L. ~1oretti, C. Vlnchesi, "KL-Cone: a 
Language for Interacting with SI-Nets", in 
Proceedings of the 8thlJCAl, Karlsruhe, 
Germany, August, 1983 . 
Cappelli A., L. Horettl, "Descrlzione dl alcuni 
fenomeni dell" Italiano con un Linguaggio dl 
Rapprcsentazlone della Conoscet*za", in Attl del 
XVII Congresso Internazionale di Stud| della 
Socteta ~ dl Ltnguis~ica Italtana, U£btno, 
Italia, Septenlber 1983. 
Caruap \[~., "Heaning and Necessity", Chicago: The 
University of Chicago Press, 1947. 
Korner S., Categorlal Frameworks, Oxford: Basil 
Blackwell, 1970. 
Hark W. S., "Indivlduallty in EL-ONE ", in 
Schmolze J.C. and R. J. Brachman (eds.), 
Proceedings of the 1981 KL-OI'E Vorkshop, Report 
n. 4842, Bolt Beranek and Ne~nan Inc., 
Cambridge (Hass.), June 1982. 
Porta O. and C. Vinehesl, "Un Slstema per la 
Rappresentazione della Conoscenza : Aspetti 
Concettuall e di Implementazione", unpublished 
Thesis , I.S.I, Pisa 1981. 
Qullllan H. R., "Semantic Hemory", in H. Uinsky 
(ed.), Semantic Information Processing, 
Cambridge (Hass.) : HIT Press, 1968. 
Sehmolze J. C. and R. J. Brachman, "Proceedings of 
the 1981 KL-ONE Workshop", Report n. 4842, Bolt 
Beranek and Newman Inc., Cambridge (Hass.), 
June 1982. 
Se~,olze J. C. and T. A. Lipkls, "Classification 
in the KL-ONE Knowledge Representation System", 
in Proceedings of the 8thlJCAl, Karlsruhe, West 
Germany, August, 1983. 
Webber B.L., "A Fomnal Approach to Discourse 
Anaphora", 1978. 
Webber B. L., "Discourse Hodel Synthesis", in A. 
Joshl, B. L. Uebber and I. Sag (eds.), Elements 
of Discourse Understanding, Cambrldge (Hass.). 
198~. 
Woods W. A., "Research in Natural Language 
Understanding", Report n. 4274, Bolt Beranek 
and ~lewman Inc., Cambrldge (Hass.), 1979. 
Woods W. A., "Abstract Algorithms and 
Arcbltectures", in Woods W. A. (ed.), Research 
in Knowledge Representation for Natural 
Language Understauding, Report n. 4785, Bolt 
Bersnek and Newman Inc., Cambridge (Hass.), 
November 1981. 
128 
