From: AAAI-83 Proceedings. Copyright ©1983, AAAI (www.aaai.org). All rights reserved. KRYPTON: Integrating Terminology Ronald Fairchild Laboratory J. Brachman for Artificial Richard Xerox Laboratory works have intuitive severely limited biguous readings. assertionally, attempted languages appeal for forming assertional power, Those based but are restricted to overcome representation based on frames system, to primitive, called descriptive terms and a logic-based the world. We here summarize one for making the two languages, have no direct assertional strictly for assertions domain-specific statements about inter- the TBox and ABox languages, $2 Two languages The TBox contains seems less than perfect of tasks required like structures of a representation system are quite popular, have limited assertional ‘instantiation’ associated when confronted capabilities and defaults), or absence of data dent predicates; capabilities, this limitation compose new predicates A productive but are limited to primitive, makes it impossible sentences [l]. to capitalize not typical of the predicate both styles of repre- 2.1 phrases that this version of) “a person with at from “a person with at most 1 child”. and disjointness the implications predicate in network calculus relationships among these terms are owns a car”, (in the logical sense) of an assertion terminological structure, style, and assertion, expressed This distinction sentation system: expressed that make up component to capture the essence of frames withdefinitional framework, without the ambiguities and possible misinterpretations In particular, expressions, common in existing the TBox supports which correspond roughly to frames, expressions, the counterparts of slots. Thus, the language a small set of Concept- and Role-forming operators. form (as in rep- In general, KRYPTON Concepts ing other Concepts in a frame- and Roles are formed by combining and Roles. For example, frame two types of expressions: the language and Role is defined by or restrictincludes an in a form of predicate yields two main components a terminological of the terminological languages.’ Concept attempted and network of the same facts (as in [3] and [12]). Instead, calculus. The language Our TBox language attempts in a compositional and strictly is KRYPTON net marriages It does not encode logical assertions like taxonomic noun and understands on the strong points of each. Over calculus/semantic 14)) nor does it use simultaneous between by (the formal terms and sentences. of indefinite the TBox and ABox languages. to syntactically While the general idea of a hybrid system is not new, distinguishes equivalent such as this one. Here we review the formal constructs indepen- actly this goal in mind. resentations the formal such as “Every person with at least 3 children and understand the past year, we have been designing and implementing an experimental knowledge representation system-called “KRYPTON’‘-with ex- before. reflect the two kinds of expres- out of old ones. view would seem to embrace in an attempt KRYPTON knowledge-(nominal) based only on their structure and not on any (domain-dependent) facts. The ABox, on the other hand, operates with the formal equivalent of assertions structures in and is disjoint The subsumption systems are usually more precise and address a much wider range of assertional sentation frame- specified, and tend to allow accidental that are being imple- for representation is subsumed least 1 child”, (e.g., they tend to be limited to with the mere presence Logic-based expression with the wide range 121. For example, the operations and how these operations such as “a person with at least 3 children”, represen- but are often imprecisely bi-conditionals of $1 Introduction tation quantified and is used mented. prover. styles of knowledge The ex- descriptions, In what follows, we will describe KRYPTON The two components predominant import. are available in can afford KRYPTON Moreover, the ABox language in more detail sions it uses to represent Each of the currently labor, in the two languages. and even universally have no special definitional among theories of domains are used as structured import. of struc- about those domains. view of the constructs pressions in the TBox language We have a functional theorem and to answer questions to take a strict knowledge in terms of a taxonomy with a first-order relationships about analytical Given its division of representational has two rep- KRYPTON one for forming and an implementation frames and its interaction terms. taxonomies tured terms and answer questions of interest logic are less limited unrelated Research one (or ‘ABox’). The TBox allows us to establish net- with am- in a new, hybrid “KRYPTON”. a frame-based fraught Intelligence these terms; the ABox allows us to build descriptive but tend to have and are often these limitations languages, face to the system, descriptions on first-order resentation or semantic Center J. Levesque for Artificial The demands placed on a knowledge representation scheme by a knowledge-based system are generally not all met by any of today’s canRepresentation Research E. Fikes Abstract didates. Intelligence Palo Alto Research Hector Fairchild and Assertion for our repre- ‘The TBox is, in large part, a distillation of KL-ONE [13], which accounts use below of terms like “value restriction” and “differentiation”. one (or ‘TBox’) and an assertional 31 for our operator ConjGenerIc ber of Concepts, junction. (‘conjoined (assuming (ConjGeneric it the expression auy num- faam i!Jr : pi man) unmarried-person Taking can also be formed operator by restricting that person ted generic’) child operator (‘value-restricted who is a man, a mother a father some number of children, TON of “a social structure this frame as a description other things, all persons”, also has an NRGeneric that restricts (PrimGeneric (ConjGeneric (NRGeneric (VRGeneric social-structure father man) (NRGeneric (VRGeneric social-structure mother father of the set of fillers person child 1 3) for “a person (VRGeneric can be defined as specializations One basic Role specialization takes a Role r and a Concept tive Role corresponding operator VRDWRoie operator would be appropriate Another Role-forming as (VRDiffRole RoleChain, operator, (‘value-restricted We have considered c, and defines the derivason, given already for defining terms child (a Role) and man (a Concept), tionally composed so that (RoleChain guage; All of the term-forming operators brother-in-law) can be composed a wide range of operators ones in the current unmarried-person Interpretation Expression (ConjGeneric (VRGeneric could be cl . . . e,) cl r ~2) (NRGeneric e r nl rz2) (PrimGeneric c i) (DecompGeneric c ii in the obvious sibling man) (NRGeneric person (RoleChain can be read as “a bachelor or, more literally, them, between conjunction value restriction “a c with between nr and n2 r’s” “a e of the i-th kind” “a c of the i-th type from the number restriction primitive Concept decomposition j-th facilities Concept Concepts “an r that is a c” Uan rn of . . . of an rrn “an r of the k-th kind” differentiation composition primitive Role (DecompRole “an r of the Cth type from the j-th [disjoint] decomposition” decomposition r i J’ die joint?) To this end, if something Concept is described of a Concept have, among (DecompGeneric by it. However, To introduce the ABox language a pair of Concepts decomposition compositionally quite different from those motivating by its As discussed sub- plete language knowledge to motivate of the operators, however, are the ones for forming TBox terms. in [2], the issue of expressive resentation the sentences from simpler ones. The behind the choice of sentence-forming concerns power in an assertional rep- is really the issue of the extent to which incom- can be represented. the standard Moreover, logical sentential this issue can be seen constructs of disjunction, (or Roles) negation operators ability to deal systematically with incomplete knowledge and to compensate for the fact that the TBox has been purged of any assertional and existential quantification ability, our ABox language relate to a more traditional the following description of the TBox language, are constructed and are given for detertwo primitive component of a would be used. To see how some of the TBox operators of frames, consider specializations of the assertional As with the expressions includes disjoint nor do they neces- KRYPTON or DecompRole) but not KRYPTON is one that is subsumed are not by definition that do have these properties, hypothetical have grand- The PrimGeneric but where no sufficient conditions 1. The TBox Language. and a man all of to be able to give necessary ‘only-if’ definitions. sarily exhaust that Concept. language person are used to form primitive or Role. A primitive superconcept, mining one wants operators decomposition” (VRDiflRole c r) (RoleChain rl . . . r,) (PrimRole r i) 2.2 The language for a definition. for specifying PrimRole whose brothers “an unmarried are men are persons whose children In many domains [disjoint] child child) 1 co)). 1 and co children”. sufficient conditions Description “a el and . . . and a c,,” “a er any r of which is a es* disjoint?) man) (VRDiffRole whose siblings that in Concepts: T&e children” are summarized Roles: (ConjGeneric This expression for the TBox lan- version defined way, as in, for example, (VRGeneric the principal child man). of uncle. used as the definition child person))). Table 1. allows Roles to be func- parent social-structure woman) 1 1) of other Roles. “an r that is a c”. Thus, this to the phrase 1 1) mother with at least 1 and not more than 3 children”. differentiation’), and KRYP- we might express it in as (‘number-restric- the cardinality for a given Role, as in (NRGeneric Roles, like Concepts, with, among who is a woman, “a cl all of whose r’s are Q’S”, as in bachelor) for “a person all of whose children The language are bachelors”. using cl, a Role r, and a Concept takes a Concept cz, and yields the term meaning (VRGeneric other Concepts has a VRGeneric KRYPTON For example, generic’) social-structure as Concepts). Concepts Roles. lea to their con- and man had appropriate that the symbols unmarried-person definitions which takes corresponding could be used to define the symbol2 bachelor This operator by assigning generic’). and forms the Concept der predicate of a family (in a operators ‘framese’): calculus 2As we will describe below, expressions can be assigned as definitions to atomic symbols. This use of defined symbols, however, is purely for the convenience of the user. 32 is structured language. compositionally between our ABox language logical one lies in the atomic symbols of a standard logical language-that (and function if any)-are symbols, like a first or- In other words, the sentence-forming are the usual ones: Not, Or, ThereExists, The major difference first order (see also 191). So to provide the sentences. and so on. and a standard The non-logical is, the predicate taken to be independent, symbols primitive, domain-dependent specifying terms. a collection Our approach, therefore, a facility In our case, there is already of domain-dependent terms, is to make the non-logical for by namely the TBox. TELL: ABox: Sentence language be the terms of the TBox language. As observed by Hayes [5] and Nilsson [lo], when the language of frames and slots is ‘translated’ into predicate calculus, place predicates respectively. are suggesting primitive the frames and slots become The main difference any theory expressed related 18 sentence As for the TBox, the TELL between what they operation the knowledge of asks whether includes the symbol, We have focused on two ASKoperations: one TBox term subsumes one TBox term is conceptually another, disjoint it The effect is to change base into one whose vocabulary defined by the term. in the ABox language).3 true? takes a symbol and associates with a TBox term (Concept or Role expression). are not to each other (independent is true. ASK: KB x SENTENCE + {yes, no, unknown} one- and two- and what we have done is that our predicates but are definitionally KB >( SENTENCE --c KB symbols of the ABox the first and the second whether from another. Schematically, this gives us $3 Operations Overall, on the components the structure 1: a TBox of roughly TBox: TELL: KB X SYMBOL X TERM + KB an ASKI: KB x TERM X TERM + {yes, no} whose predicates come from the the names of the TBox terms so ASK:!: KB x TERM x TERM -+ {yes, no} of KRYPTON can be visualized KL-ONGish terms organized ABox of roughly first-order sentences TBox, and a symbol table maintaining taxonomically, that a user can refer to them. However, this is a somewhat picture since it suggests directly. that users can manipulate By symbol, I mean as in Figure Doea term1 Is terrnl misleading In fact, a user does not get access to either a network is a fixed set of operations All interactions mediated subsume disjoint term27 term27 Jrom these structures in the The TBox ASK operations TBox or to a collection of sentences in the ABox. What a user does get instead term. the domain-dependent over the TBox and ABox languages. between a user and a KRYPTON knowledge allow a user to inquire about the meaning of terms being used (without is known about the world). base are to understand In addition, the non-logical also folding in what the ABox uses these operations terms in the sentences it processes. by these operations. Of course there have to be additional edge base. For instance, no way of getting ASK operations the ones that we have mentioned other than a yes/no answer. In the case of the ABox, we have to be able to find out what individuals have a given property; in the TBox, there has to be some way of getting TI!OX the definitions (e.g., the fact that necessarily 3). It is important knowledge taxonomy 1. KRYPTON overview. of angles of a triangle system is completely and ASK operations. or a set of first-order the interface Figure the number from and disJointness is to stress that the service provided by KRYPTON as a representation of the TELL the information that is not provided by the subsumption operations on a knowlso far provide provided the notions of a clauses in normal form are not part of by the system 4 The actual symbolic to realize the TELL used by KRYPTON specified by a definition In particular, structures and ASK operations are not available to the user. While it might be useful to think of a knowledge base as structured in a certain way, this structure can only be inferred base can be divided into from the system’s two groups: the TELL operations, used to augment a knowledge base, and the ASK operations, used to extract information. In either case, The operations on a KRYPTON knowledge allow finer-grained the operation the operations can be definitional and asserts that it is true. the knowledge operation speaking, is to change defined in the TBox. 3The status of function base and the vocabulary Schematically, symbols and predicate can be told (involving to TELL) to other terms in the of this approach that bases,5 but again it will be used to implement expressions to knowledge conceptually language). representa- between what a system in the language and what it has to actually tures in the implementation remember used as arguments (involving data struc- This allows us to consider an these operations 41n [s], we present a definition of TELL and ASK that is completely independent of how the knowledge is represented in a knowledge base and is based instead on symbols with more than two argu- the set of worlds that are compatible ments is unclear at present. There is no problem incorporating them as primitive8 into the ABox language; the issue is what facilities there should be for relating them definitionally property tion is that there is a difference used in the sentence, as we can describe to be deduced, that count, not the data structures One interesting of the world implies that sentence. The corresponding ASK operation takes a sentence and asks if it is true. The result is determined on the basis of the current theory held by the knowledge new operations to be made among knowledge them. takes an ABox sentence The effect, roughly base into one whose theory One might consider distinctions allowing even more of its structure or assertional. In terms of the ABox, the TELL behavior. with what is known. ‘One possibility, for example, is an ABox ASK operator form of limited inference over what is known. TBox. 33 that only performs some interface notation plementation that is, in some sense, more expressive the translation. supply In [S], a modal arguments represented ‘auto-epistemic’ to TELL and yet the resulting in first-order than the im- that the diflerence can be accounted one, provided for in language is used to knowledge is always in efficiency over simply asserting To this end, we are developing that take into account the meaning extensions dependencies postulates as aMoms~T of standard inference rules among predicates derivable from TBox definitions. terms. For example, the reasoning of a standard resolution theorem prover depends $4 Building KRYPTON Having discussed the desired functionality edge representation building on noticing that an occurrence of @(z) in one clause is inconsistent with 1 $(z) in another. Given that realization, the two clauses can be used to infer a resolvent clause. The scope of this inference of the KRYPTON knowl- system, we now sketch in general terms how we are a system with these capabilitites. rule can be expanded The first thing tency of two literals.8 That is, since triangle and rectangle are disjoint, and rectangle(z) situation is that because of the expressive very general reasoning Specifically, we cannot typical strategies of the ABox power of the assertional language, will be needed to answer questions. representation to the special-purpose methods to locate a representational object of the cow-in-the-field an instantiation standing Concept), cows and that at least one of his animals The second predicate point worth ally inconsistent the ABox is that but 4.2 literals can still be resolved provided polygon of those once told that Elsie is a cow, the ABox should steps in a deduction, to compute of the first one since they depend on how the the issue here is that the ABox predicates are not simply unconnected primitives (as in firstorder logic), so that if we want to use standard first-order reasoning we have to somehow make the connections implied by the TBox. the simplest way to make the TBox-ABox connection strategy performed reasonably cow we could automatically first-order reasoning after defining assert sentences over the the Concept saying that every cow is an animal and that cows are not bulls, as if these were observed about the world. As far as the ABox is concerned, term would be no more than the assertion the definition of a ‘meaning In some sense, this would yield a ‘hybrid’ system discussed facts stating the Absolutely most important by our TBox language l), we stand gain %deed, by far the most common rendering of definitions in systems based on firstorder logic is as assertions of a certain form (universally quantilied bi-conditionals), a treatment which fails to distinguish them from the more arbitrary facts that happen to have the same logical form. route. can be limit on the TBox operations One can imagine wanting a providing a set of term-forming of getting facilities a usable our predicates like those we have already discussed-see at least a chance algorithm Table while that have been found useful in AI applications. The situation of term-forming language is far from resolved, of term subsumption operators. without however. The computational seems to be very sensitive to the choice For example, it appears that given our TBox the VRDifITtole I? a significant postulate much less an efficient one. By restricting initional and provides are as hard as theorem to what might be called the ‘frame-definable’ algorithm will be O(n”) at worst; however, the problem is as difficult information, nothing will be gained by our itself. same set of facts. Our goal, however, is to develop an ABox reasoner that avoids such redundancies, maintains the distinction between defand assertional between quickly with respect to the ABox. would then be possible, complexity like the kind in [3] and 1121, since we would have two notations has to be information language that would allow arbitrary ‘lambda-definable’ predicates to be specified. The trouble is that no complete algorithm for subsumption of a postulate’.6 an ABox reasoner and disjointness if the TBox operations The first and perhaps is provided (in terms of operators standard if the TBox is it should answer, in three steps to ensure that the TBox operations the ABox, and then to perform For example, Thus, we could just as well have gone the meaning TBox language theory. the conditionthat we include we have to be very careful about how long it takes is to cause the act of defining a term in the TBox to assert a sentence in expanded in the resolvent. is disjoint from -rectangle, that information. implementation We are taking resultant only when all the angles of In such cases, the clauses containing able to access TBox subsumption not logical consequences Conceptually, with -rectangle(z) has The literal Making a TBox know that Elsie is an animal and is not a bull. However, these facts are techniques, assume that rectangle angle right-angle). Since we take the point of view that if the are indeed TBox terms, then cow is defined in the TBox. In general, polygon also imply effect, “only when all the angles are right angles”. proving; Concept For example, of the condition asked whether has escaped into the field. needs to have access to the TBox definitions For example, is inconsistent The TBox definitions since, among other Smith owns nothing about symbols of the ABox language the ABox reasoner terms. noticing by the fact that inconsistencies. 5 are right angles. sub- are inconsistent. for it (i.e., we may not know all the cows or even how many there are. Yet, we may very well have been told that polygon(z) and since polygon and rectangle(z) been defined as (VRGenerIc the negation systems. to find out if there is a cow in the field, it will not For example, things, an implementation limit ourselves of frame-based be sufficient about are inconsistent; ‘polygon(z) is complicated ‘conditional’ to notice informa- the inconsis- triangle(z) Making an ABox and disjointness means of recognizing sumes rectangle, 4.1 by using subsumption tion from the TBox as an additional operator, the term subsumption with the VRDifIRole operator, as propositional theorem proving ‘There are already precedents in the theorem-proving literature (see [8] and Ill]) for using special information about subsumption and disjointness of predicates as a way of guiding a proof procedure. 8See [IS] for a similar approach to augmenting resolution by ‘building-in’ a theory. As a second step towards fulfilling the TBox, we have adopted sumption relationships data structure. this efficiency a caching for symbols an explicit We are also developing methods cache to include both absolute and conditional about TBox terms. The key open question is how to determine conditional a useful subset relationships regarding determines the new symbol Because incrementally the taxonomy reflects R., and Hendrix, IJCAI-77, 151 the no- I61 relation- Cambridge, its correct position. the relationship Levesque, H. J., “A Formal Treatment 1981. Also available 3, Fairchild The Laboratory Alto, CA, February, that-an implementation and the definition taxonomy strategy. is that and and of Incomplete Knowledge Science, University as FLAIR Technical for Artificial Intelligence Report of No. Research, Palo 1982. “Some Results on the Complexity Levesque, H. J., Language,” of Subsump- in preparation, 1983. stra- it is precisely 181 The meaning of the TBox language of the TBox operators in Proc. of a PI and classification Knowledge System,” Hayes, P. J., “The Logic of Frames, n in Frame Conceptions Text Understanding, Metzing, D. (ed.), Walter de Gruyter Toronto, we can thing to notice about this implementation based on a taxonomy Deduction MA, 1977, 23.5-246. tion in a Frame-based One very important “A Network-Based G., and its Natural Bases,” Ph. D. thesis, Dept. of Computer symbol to all the other defined symbols. tegy ing Intelli- Co., Berlin, 1979, 46-61. for which it is still about Fikes, Representation scheme is that the symbol taxonomy more and more informed Artificial [14], wherein a a partial-ordering, move the symbol down towards overall effect of this classification slowly becomes [4] this of possible the subsumption and each symbol for ProbIem-Sola Information,” gence 16, 3 (1981), 225-255. these extensions KL-ONE “A Common Representation E., and Language-Comprehension information of the large number Charniak, of for extending an efficient TBox, we have adopted process sequentially unknown. PI that could be defined between the symbols. As a final step towards background taxonomy disjointness tion of a classifier, much like the one present in ship between ffor defined by the user in a tree-like We, in effect, are maintaining the defined symbols. requirement scheme in which we store sub- McSkimin, J. R., and Minker, J., Semantic Network Networks: do not depend at all on the or on how well the classifier is doing at some point. Representation Findler, PI Calculus Based Searching,” in Associative and Use of Knowledge by Computers, N. V. (ed.), A ca d emit Press, New York, 1979, 205238. Moore, R. C., “The Role of Logic in Knowledge and Commonsense g5 Conclusion “A Predicate for Deductive in Proc. Reasoning,” Representation AAAI-82, Pittsburgh, 1982, 428433. KRYPTON The based system vantages of each. tured descriptions organized supports component standard logic for expressing first-order two components interact through An implementation of a As of this writing, terminological operators, knowledge. 1131 the of the [14] JACM with in 1151. IEEE PI Computer, to Representation September, Brachman, R. J., Knowledge Representation,” and H. J., “KRYPTON: PI System,” to appear in 1983. Levesque, Closure in First-Order “Knowledge Calculus: How to Have Your Cake and Eat it Too,” in Proc. AAAI-82, Pittsburgh, Schmolze, Representation and Predicate 1982, 193196. J. G., and Brachmnn, Laboratory Languages Workshop,” R. J., eds., “Proceedings FLAIR Technical for Artificial Intelligence Report Research, of No. Palo Schmolze, J. G., and Lipkis, T. A., Stickel, M. E., “A Nonclausal Program,” “Classification System,” in Proc. Connection-Graph in Proc. AAAI-82, in the KL- IJCAI-83, Resolution Pittsburgh, 1982, 229-233. J., Fikes, R.E., andLevesque, Approach and Domain Knowledge Representation Karlsruhe, W. Germany, 1983. References A Functional Tioga Rich, C., Theorem-Proving Brachman,R. Intelligence, 27, 2 (1980), 235-249. ONE 1151 [l] of Artificial Alto, CA, May, 1982. methodol- its interaction “Equality R., the Second KL-ONE is under- the operations investigating Reiter, Principles Co., Palo Alto, CA, 1980. 4, Fairchild system in Interlisp-D described These it. using the taxonomy/classification a version of the theorem-prover [12] each interface; used to implement KRYPTON ogy discussed above, and are currently of utilizes the power of incomplete N. J., Databases,” Lomponent some domain term-forming component we have implemented component Ill1 of struc- the assertional Nilsson, Publishing and an assertional the formation a functionally-specified user has no access to the structures [lo] is a distilled version of frames, by a set of compositional The assertional frame- the disad- the representation to be used to characterize with clear import. way. separates taxonomically; The terminological characterized to integrate a terminological component allows these descriptions interest. KRYPTON components: The terminological an attempt in such a way as to minimize To do this, task into two distinct one. represents and logical facilities H. J., in Proc. “Competence AAAI-82, in Pittsburgh, 1982, 189192. 35 Stickel, M. E., “Theory Resolution: Building-In Nonequational Theories,” in Proc. AAAI-83, Washington. D. C., 1983.