Semantic Web
SeanBechhofer, IanHorrocks,CaroleGoble, andRobertStevens
InformationManagementGroup,DepartmentofComputerScience,
UniversityofManchester,
OxfordRoad,ManchesterM139PL,UK
{seanb,horrocks,carole,stev ensr} @cs. man. ac.uk ,
http://img.cs.man.ac.uk
Abstract. Ontologies will play a pivotalr^oleinthe \SemanticWeb",
where they will provide a source of precisely dened terms that can
becommunicatedacrosspeopleand applications.OilEd,isanontology
editorthathasaneasytouseframeinterface,yetatthesametimeallows
users to exploit the full powerof anexpressive webontology language
(OIL).OilEdusesreasoningtosupportontologydesign,facilitating the
developmentofontologiesthatarebothmoredetailedandmoreaccurate.
1 Introduction
Ontologieshavebecomeanincreasinglyimportantresearchtopic.Thisisaresult
bothoftheirusefulnessinarangeofapplicationdomains[1{3],andofthepivotal
r^olethattheyaresetto playinthedevelopmentoftheSemanticWeb
TheSemanticWebvision,asarticulatedbyTimBerners-Lee[4],isofaWeb
in which resources are accessible not only to humans, but also to automated
processes, e.g., automated \agents" roaming the web performing useful tasks
suchasimprovedsearch(intermsofprecision)andresourcediscovery,informa-
tion brokering and information ltering. The automation of tasks depends on
elevating thestatus of the web from machine-readable to somethingwe might
call machine-understandable. The key ideais to havedata on theweb dened
andlinkedinsuchawaythatitsmeaningisexplicitlyinterpretablebysoftware
processesratherthanjust beingimplicitlyinterpretablebyhumans.
To realise this vision, it will be necessaryto annotate web resources with
metadata(i.e.,datadescribingtheircontent/functionality).Standardisationpro-
posalsforannotationlanguageshavealreadybeensubmittedtotheWorldWide
WebConsortium(W3C),inparticularRDF(ResourceDescriptionFramework)
andRDFSchema(see[5]foradiscussionofther^olesoftheselanguagesandof
XML/XMLSchema).However,suchannotationswillbeoflimitedvaluetoauto-
matedprocessesunlesstheyshareacommonunderstandingastotheirmeaning.
Ontologies,can helptomeetthisrequirementbyprovidinga\representationof
asharedconceptualisation of aparticular domain"that canbecommunicated
representationlanguage:ittalksaboutclassesandproperties(binaryrelations),
range and domain constraints (on properties), and subclass and subproperty
(subsumption)relations.However,RDFSisarelativelyprimitivelanguage(the
aboveisanalmostcompletedescriptionofitsfunctionality),andmoreexpressive
power would clearly be necessary/desirable in order to describe resources in
suÆcientdetail.Moreover,such descriptionsshould beamenableto automated
reasoning iftheyaretobeused eectivelybyautomatedprocesses.
These considerations have led to the development of OIL [7], an ontology
languagethat extendsRDFS withamuchrichersetof modellingprimitives.A
similar RDFS based web ontologylanguagecalled DAML wasbeendeveloped
aspartoftheDARPADAMLproject[8]andthetwolanguageshavenowbeen
mergedunder the nameDAML+OIL 1
. OIL hasaframe-like syntax, which fa-
cilitates tool building, yet canbemapped ontoan expressivedescription logic
(DL),whichfacilitatestheprovisionofreasoningservices.OilEdisanontology
editingtoolforOIL(andDAML+OIL)thatexploitsboththesefeaturesinorder
toprovideafamiliar andintuitivestyleofuserinterfacewiththeaddedbenet
ofreasoningsupport.Its main noveltyliesin the extensionof theframeeditor
paradigmto dealwithaveryexpressivelanguage,and theuseofahighlyopti-
mised DLreasoningengineto providesound andcomplete yet still empirically
tractablereasoningservices.
Reasoning with terms from deployed ontologies will be important for the
SemanticWeb, but reasoning support is also extremely valuableat the ontol-
ogy design phase, where it canbe used to detect logically inconsistent classes
and to discoverimplicit subclass relations.This encouragesa moredescriptive
approach to ontologydesign, with thereasonerbeingused to inferpartof the
subsumption lattice (see the case study presented in Section 4); the resulting
ontologiescontainfewererrors,yet providemoredetaileddescriptionsthatcan
beexploitedbyautomatedprocessesintheSemanticWeb.Finally,reasoningis
of particular benet when ontologies are large and/or multiply authored, and
alsofacilitatesontologysharing,mergingandintegration[9];considerationsthat
willbeparticularly importantin thedistributed webenvironment.
2 Oil and DAML+OIL
The development of OIL resulted from eortsto combine the best features of
frameandDLbasedknowledgerepresentationsystems,whileatthesametime
maximising compatibility with emerging web standards. The intention was to
designalanguagethatwasintuitivetohumanusers,andyetprovidedadequate
expressivepowerforrealisticapplications(manyearlyDLsfailedonthissecond
count|see[10]).
The resulting language combines a familiar frame like syntax (derived in
partfrom theOKBC-liteknowledgemodel[11]), withthepowerand exibility
1
andinverseslots,generalaxioms,etc.).Thelanguageisdenedasanextension
ofRDFS, therebymaking OIL ontologies (partially)accessibleto any \RDFS-
aware"application.
Theframesyntaxislessdauntingtoontologists/domainexpertsthanaDL
stylesyntax,anditfacilitatesamodellingstyleinwhichontologiesstartoutsim-
ple(in terms oftheir descriptivecontent)and are graduallyextended,bothas
thedesignitselfisrenedandasusersbecomemorefamiliarwiththelanguage's
advancedfeatures (seeSection4). Theframeparadigmalsofacilitatesthecon-
structionandadaption oftools,e.g.,theOntoEditand Protegeeditorsandthe
Chimaera integration tool are all beingadapted to use OIL/DAML+OIL [12,
13,9].
Ontheotherhand,basingthelanguageonanunderlyingmappingtoavery
expressiveDL(SH Q)providesawelldened semanticsandaclearunderstand-
ingofitsformalproperties,inparticularthattheclasssubsumption/satisability
problemisdecidableandhasworstcaseExpTimecomplexity[14].Themapping
also provides a mechanism for the provision of practicalreasoning services by
exploitingimplementedDLsystems,e.g.,theFaCTsystem[15].
OILextendsstandardframelanguagesinanumberofdirections.Oneofthe
keyideasisthatananonymousclassdescription,orevenbooleancombinations
of class descriptions, can occur anywhere that a class name would ordinarily
beused,e.g.,in slotconstraintsand inthelist ofsuperclasses.Forexample,in
Figure 1(which usesOIL's\humanreadable"presentationsyntaxrather than
themoreverboseRDFSserialisation),aherbivoreisdescribedasananimalthat
eatsonly plants orpart-ofplants.Pointsto noteare that universally quantied
(value-type) and existentiallyquantied (has-value) slot constraintsareclearly
dierentiated, and that the constraint on theeats slot is adisjunction, one of
whosecomponentsisananonymousclassdescription(inthiscase,just asingle
slotconstraint).Inaddition,itisassertedthat thepart-ofslotistransitive,and
thatitsinverseistheslothas-part.Furtherdetailsofthelanguagewillbegiven
inSection 3,andacompletespecicationcanbefoundin[7].
slot-def part-of
subslot-of structural-relation
inversehas-part
properties transitive
class-def denedherbivore
subclass-of animal
slot-constraint eats
value-typeplantOR
slot-constraint part-ofhas-value plant
Fig.1.OILlanguageexample
OilEdis asimpleontologyeditor that supports the constructionof OIL-based
ontologies. The basicdesign has beenheavily inuencedby similar tools such
as Protege [13] and OntoEdit [12], but OilEd extends these approaches in a
numberofways,notablythroughanextensionofexpressivepowerandtheuse
ofareasoner.
However, OilEd is not intended as a replacement for such tools|the cur-
rentimplementation of OilEdis intendedprimarily asa prototypeto test and
demonstrate novel ideas,and compromiseshave been made in the design and
implementation. For example, the tool does not provide key functionality for
collaborativeontologydevelopmentsuchasversioning,integrationandmerging
ofontologies.Similarly,thepowerfultailorabilityandknowledgeacquisitionas-
pectsoftoolssuchasProtegehavebeenignoredcompletely.Rather,thedesign
hasconcentratedondemonstratinghowtheframeparadigmcanbeextendedto
dealwithamoreexpressivemodellinglanguage,andhowreasoningcanbeused
tosupportthedesignandmaintenanceofontologies.
3.1 OilEdFunctionality
Basicfunctionalityallowsthedenitionanddescriptionofclasses,slots,individ-
ualsand axiomswithin an ontology.Ingeneral, editingfunctions are provided
through graphical means|mousedriven drop down menus, toolbars and but-
tons.We will notprovide adetailed descriptionof thegraphical userinterface
here,asit isrelativelystandard(see Figure2, which providesascreen shotof
theeditorsclassdenitionpanel).Instead,wewilldiscussthenovelfunctionality
oeredbythetool.
Frame Descriptions The central component used throughout OilEd is the
notionofaframedescription. Thisconsistsofacollectionofsuperclassesalong
with a list of slot constraints. This is similar to other frame systems. Where
OilEddiers,however,isthat whereveraclassname canappear,arecursively
dened,anonymousframedescriptioncanbeused.Inaddition,arbitraryboolean
combinationsofframesorclasses(usingand,orandnot)canalsoappear.This
isin contrastto conventional framesystems,where in general,slot constraints
andsuperclassesmustbeclassnames.
As well as being able to assert individuals as slot llers, several types of
constraintson slot llers can be asserted (these kinds of constraint are some-
timescalled facets).These include value-typerestrictions(all llersmust be of
aparticular class), has-valuerestrictions(there must be at least one llerof a
particularclass),andexplicitcardinalityrestrictions(e.g.,atmostthreellersof
agivenclass).Eachconstrainthasaclearlydenedmeaning,removingthecon-
fusionpresentin someframesystems,where, forexample,itisnotalwaysclear
whether thesemanticsof aslot-constraintshould beinterpreted asauniversal
orexistentialquantication.
Class DenitionsA class denition species the class name, along with an
isdenedorprimitive.Ifdened,theclassistakentobeequivalenttothegiven
description(necessaryandsuÆcientconditions).Ifprimitive,theclass istaken
tobeanexplicitsubclassofthegivendescription(necessaryconditions).Inthe
specicationoftheOILlanguage,classescanhavemultipledenitions.InOilEd,
thisisdisallowedforimplementationreasons.Insteadclassesmusthaveasingle
denition, but the sameeect can be achieved through theuse of equivalence
axiomsasdiscussedbelow.Ontologiesusingmultipledenitionscanbereadby
the tool. The rst denition encountered will be used as the class denition,
withanysubsequentdenitionsbeingtranslatedtotheappropriateaxioms.
Slot DenitionsAslot denition givesthenameof theslot andallowsaddi-
tional propertiesof theslot tobeasserted,e.g.,thenamesof anysuperslotsor
inverses.Ifrisasuperslotof s,thenanytwoobjectsrelatedviasmustalsobe
relatedviar(i.e.,s(a;b)!r(a;b));ifrisaninverseofs,thenaisrelatedtobvia
sibisrelatedtoaviar(i.e.,s(a;b)$r (b;a)).Domainandrangerestrictions
ona slot canalsobespecied.Forexample,wecanconstrain therelationship
parentto have bothdomain and range person, asserting that only personscan
have,andbe,parents.Aswithclass descriptions,thedomainandrangerestric-
tionscan bearbitrary class expressions such as anonymous frames orboolean
global, andapplytoeveryoccurrenceoftheslot,whether explicitorimplicit.
Aslotrcanalsobeassertedtobetransitive(i.e.,r (a;b)andr(b;c)!r (a;c)),
functional(i.e.,r (a;b)andr(a;c)!b=c)orsymmetric(i.e., r(a;b)!r (b;a)).
All assertions made aboutslots are used by the reasoner, and may induce
hierarchicalrelationshipsbetweenclasses,e.g.,asaresultofdomain andrange
restrictions.
AxiomsAnother areawhere theexpressive powerof OIL/OilEdexceedsthat
oftraditionalframelanguages/editorsisin thekindsofaxiom thatcanbeused
to assert facts aboutclasses and their relationships.As well asstandard class
denitions (which are really arestricted form of subsumption/equivalence ax-
iom),OilEdaxiomscanalsobeusedtoassertthedisjointnessorequivalenceof
classes(with the expected semantics) along with coverings. A coveringasserts
thateveryinstanceofthecoveredclassmustalsobeaninstanceofatleastone
ofthecoveringclasses.Inaddition,coveringscanbesaidtobedisjoint,inwhich
case everyinstance of the coveredclass mustbe an instance of exactlyone of
thecoveringclasses.
Again,these axiomsarenotrestrictedto classnames,but caninvolvearbi-
traryclass expressions(anonymous framesorboolean combinations).This isa
verypowerfulfeature,andisoneofthemainreasonsforthehighcomplexityof
theunderlyingdecisionproblem.
IndividualsLimitedfunctionalityisprovidedtosupporttheintroduction and
descriptionof individuals|the intention within OilEd is that such individuals
are foruse within class descriptions,rather thansupportingthe production of
largeexistentialknowledgebases (itissupposed that RDF/RDFSwill beused
directly for this purpose). As an example, we may wish to dene the class of
Italians as being all those Persons who were born in Italy, where Italy is not a
classbutanindividual.
As theFaCTsystem doesnotsupport reasoningwith individuals,they are
treated(forreasoningpurposes)asdisjointprimitiveclasses.Thisisnotanideal
solutionasitdoesleadtosomeinferencesbeinglost,inparticularthoseresulting
fromtheinteractionbetweenindividualsandmaximumcardinalityconstraints.
E.g., it would not be possible to infer that Persons who are citizens of Italy,
andof nootherCountry, arecitizensof atmostoneCountry. Workis currently
underwayto extendtheFaCTreasonertodealexplicitlywithsuch individuals,
sothat completeinferencecanbeprovided.
ConcreteDatatypesConcretedatatypes(stringandintegers),alongwithex-
pressionsconcerningconcrete datatypes(suchasmin, maxorranges)canalso
beusedwithinclassdescriptions.However,theFaCTreasonerdoesnotsupport
reasoningoverconcretedatatypes,andatpresentOilEdsimplyignoresconcrete
datatype restrictions when reasoningaboutontologies. The theory underyling
concretedatatypesis,however,wellunderstood[16],andworkisalsoinprogress
for the denition of data types. These are not fully supported in our current
versionofOilEd.
3.2 Reasoning
Inadditiontotheextendedexpressivitydiscussedabove,OilEd'sprincipalnov-
elty isin its useof reasoningto check classconsistency andinfersubsumption
relationships. Reasoning services are currently provided by the FaCT system,
but in principal any reasonerwith the appropriate functionality/connectivity
couldbeused.
FaCTisaDLclassierthatoerssoundandcompletereasoning(satisabil-
ity,subsumptionandclassication)fortwoDLs:SH FandSH Q. FaCT'smost
interestingfeaturesareitsexpressivelogic(inparticulartheSH Qreasoner),its
optimisedtableauximplementation(whichhasnowbecomethestandardforDL
systems),anditsCORBAbasedclient-serverarchitecture[15].
TheSH QlanguagecancompletelycaptureOIL ontologies, withtheexcep-
tionoftworecentlyadded features:concrete datatypes(strings, numbers,etc.)
and named individuals in class descriptions. As mentioned above, individuals
canbedealtwithbytreatingthemaspairwisedisjointatomicclasses(although
with somelossofinferentialpower), whileextendingFaCT todeal withOIL's
concretedatatypesshould berelativelystraightforward.
FaCT's optimisations arespecically aimedat improving thesystem'sper-
formancewhenclassifyingrealisticontologies. Theseoptimisations leadtoper-
formance improvements of several orders of magnitude when compared with
older DL and modal logic reasoners, and make the use of reasoning support
feasableinspiteofthediscouragingworstcasecomplexityoftheunderlyingde-
cisionproblem(ExpTime).Theperformanceimprovementisoftensogreatthat
itis impossibleto measureprecisely asunoptimisedsystemsarevirtually non-
terminatingwith ontologies that FaCT is easily ableto deal with [15].Taking
alargemedicalterminologyontologyasanexample[17],FaCTisabletocheck
theconsistencyof all 2,740classes anddetermine thecomplete classhierarchy
in about45seconds of(700MHzPentiumIII) CPU time;unoptimisedsystems
have been run for several weeks without their completing even a single class
consistencytest.
In the current version of OilEd, reasoning is performed on a \single-shot"
basis,i.e.,atsomesuitablepointtheuserconnectstothereasonerandrequests
vericationoftheontology.ConnectionisviaFaCT'sCORBAbasedclient-server
interface,which hastheadvantagethat FaCT servers(s)canberunningeither
locally or remotely,and can provideaservice to manyOilEdusers. Moreover,
the FaCT system has reasoning engines for both SH Q and SH F knowledge
bases,and if both servicesare available the usercan choose to connect to the
faster SH F reasonerto verify anontologythat doesnotinclude either inverse
slotsorcardinalityconstraints.Thecurrentimplementation simplyinformsthe
userifthisisappropriate;futureenhancementswillinclude automaticselection
Whenvericationisrequested,theontologyistranslatedintoanequivalent
SH Q(orSH F)knowledgebase andsentto thereasonerforclassication[18].
OilEdthenqueriestheclassiedknowledgebase,checkingforinconsistentclasses
andimplicitsubsumptionrelationships.Theresultsarereportedtotheuserby
highlightinginconsistentclasses andrearranging the class hierarchy displayto
reect any changes discovered.FaCT/OilEddoesnotprovide any explanation
ofitsinferences,althoughthiswould clearlybeusefulinontologydesign[19].
Figures 3and 4showthe eects ofclassicationon (part of)thehierarchy
derived from the TAMBIS ontology (see Section 4). When verifying the on-
tology, anumber of newsubsumption relationships are discovered(due to the
classdenitions in themodel). Inparticularwecansee that,afterverication,
holoenzymeisnotonlyanenzyme,butalsoaholoprotein,andthatmetal-ionand
small-moleculearebothsubclassesofcofactor.
During subsequent editing, changes to the ontologyare notcommunicated
tothereasonerinstantaneously,butonlywhenexplicitlyrequestedbytheuser.
thesimpleinteractionmodel describedherewasconsideredappropriateforthe
initialprototype.
3.3 Export
AlthoughOilEd isprimarily intendedasan editorfor OIL ontologies, thetool
willexporttoanumberofformats.These includeOIL Standard(the \human-
readable"presentation format forOIL that wasused in Figure 1), OIL-RDFS
(OIL'sstandardRDFSserialisation)andDAML+OIL(alsoRDFS).Inaddition,
ontologiescanbeexportedasHTML,facilitatingviewingoftheontologywithout
the tool and class hierarchies generated by the classier can be exported as
graphsforviewingwithAT&T's Dotty 2
application.
ByexportingontologiesasRDFS, itisenvisagedthat \RDFS-aware"appli-
cations will be able to read and interpret OIL ontologies even if they are not
fully\OIL-aware".Ofcourse,suchapplicationswouldbeunabletomakeuseof
alloftheinformationinthemodel,butmaybeabletouse,forexample,thesub-
classhierarchieswithintheontology.Inordertofacilitatethis,OilEdallowsthe
2
tologybeforeexport, thusmakingthisinformationavailableto non-OILRDFS
applications,orevenOIL-awareapplicationsthat donotemployreasoning.
4 Case Study: the TAMBIS Ontology
The r^ole of ontologies in bioinformatics (the discipline of applying computing
to molecular biology) hasbecome prominent in the last few years. Ontologies
are usedas amechanismfor expressing and sharingcommunityknowledge,to
dene common vocabularies (e.g., for database annotations), and to support
intelligentqueryingovermultiple databases[20].TAMBIS(TransparentAccess
toMultipleBioinformaticInformationSources)isamediationsystemthatuses
anontologytoenablebiologiststoaskquestionsovermultipleexternaldatabases
usingacommonqueryinterface.TheontologyiscentraltotheTAMBISsystem:
it provides a model that queries can be formed against, it drives the query
formulation interface, it indexes the middleware wrappers of the component
sources,anditsupportsthequeryrewritingprocess[21].TheTAMBISontology
(TaO) covers the principal concepts of molecular biology and bioinformatics:
macromolecules;theirmotifs,theirstructure,function,cellularlocationandthe
processes in which they act. It is an ontologyintended for retrieval purposes
ratherthan hypothesis generation,soit isbroadand shallowratherthan deep
andnarrow[20].
TheTaOwasoriginallymodelledintheDLGrail[17].Itwassubsequently
migratedtoOILinorderto(a)exploitOIL'shighexpressivitysoastomaintain
abetter delitywith biological knowledge asit is currently perceived; (b) use
reasoning support when building and evolving complex ontologies where the
knowledge is dynamic and shifting; and (c) be able to deliver the TaO as a
conventionalframeontology(withallsubsumptionsmadeexplicit),thusmaking
itaccessibleto awiderrangeof(legacy)applicationsandcollaborators.
Theapproachtodevelopingtheontologywasdirectlyinuencedbytherange
ofexpressivitythatOILaords,andthecapabilitiesofOilEditself,particularly
itsreasoning facilities.Themodellingphilosophy wasto bedescriptive,i.e., to
modelpropertiesandallowasmuchaspossibleofthesubsumptionlatticetobe
inferredbythereasoner.Thedesignmethodologywastorstconstructabasic
framework of primitive foundation classes and slots, working both top down
and bottom up, mainly using explicitly stated superclasses.The ontologywas
thenincrementallyextendedandrenedbyaddingnewclasses,elaboratingslot
llersandconstraints,and \upgrading"todened classeswhereverpossible,so
thatclassspecicationsbecamesteadilymoredetailed andmoreaccurate.This
process wasguided by subsumption reasoning|whenelaborating or changing
classes,thereasonercouldbeusedtocheckconsistencyandtoshowtheimpact
sentation syntax) that we will use to illustrate this methodology. 3
Originally,
holoprotein,enzyme andholoenzymewereallprimitiveclasses,withnoslotcon-
straints,andanexplicitlyassertedclasshierarchy:holoproteinandenzymewere
subclassesofprotein, andholoenzyme wasasubclassofenzyme. Duringtheex-
tensionandrenementphase,thepropertiesofthevariousclassesweredescribed
in moredetail: it was assertedthat aholoproteinbindsa prosthetic-group, that
anenzyme catalyses areaction, and that aholoenzyme bindsaprosthetic-group.
Severaloftheclasseswerealsoupgradedtobeingdenedwhentheirdescription
constitutedbothnecessaryandsuÆcientconditionsforclass membership, e.g.,
a protein is a holoprotein if and only if it binds a prosthetic-group. This allows
thereasonerto inferadditionalsubclassrelationshipsw.r.t.holoprotein, andin
particular that holoenzymeis asubclass of holoprotein. This latterrelationship
probablywouldhavebeenmissediftheontologyhadbeenhandcrafted.
class-def protein
class-def dened holoprotein
subclass-of protein
slot-constraintbindshas-value prosthetic-group
class-def dened enzyme
subclass-of protein
slot-constraintcatalyseshas-value reaction
class-def dened holoenzyme
subclass-of enzyme
slot-constraintbindshas-value prosthetic-group
class-def dened cofactor
subclass-of (metal-ionor small-molecule)
disjointmetal-ionsmall-molecule
Fig.5.SimpliedfragmentofTAMBISontology
The extension and renement phase also included the addition of axioms
asserting disjointness, equality and covering, further enhancing the accuracy
of the model. Referringagain to Figure 5, ourbiologist initially assertedthat
cofactorwasasubclass of bothmetal-ionand small-molecule(a commonconfu-
sionoverthesemanticsof'and'and'or')ratherthanbeingeitherametal-ionora
small-molecule.Subsequently,whenitwasassertedthatmetal-ionandsmall-molecule
are disjoint, thereasoner inferred that cofactor was logicallyinconsistent, and
themistake was rectied.Modelling mistakessuchas these litter bioontologies
craftedbyhand.
Other advantagesderivedfrom theuseofOilEdincluded:
3
The complete ontology can be found at http://img.cs.man.ac.uk/stev ens/
language, made ontology developmentmuch less daunting to our biologist
thanwritingSH Qlogicexpressionswouldhavebeen.
{ ClipboardfacilitiesprovidedbyOilEdallowed(partsof)framestobecopied
andpasted,makingiteasytoexperimentwithnewdenitionsandtomain-
tain a consistent modelling style. E.g., coenzymeA-requiring-oxidoreductase
wasbuiltbycopyingnad-requiring-oxidoreductaseandchangingtheconstraint
on thebindsslot from nadto coenzymeA. Thereasonerthenautomatically
migrated theclassfrom beingasubclass ofholoenzymeto beingasubclass
ofcoenzyme-requiring-enzyme.
{ Class denitions canbe assimple aspossible yet ascomplexasnecessary.
PartsoftheTaOaresimplyprimitiveframesandslots;otherpartsarevery
elaborate andexploitthefullexpressivepoweroftheOILlanguage.
{ In TAMBIS, the ontology is managed by an ontology server that makes
fulluseoftheclassdenitions,e.g.,toclassifyusergeneratedqueryclasses.
However,beingabletodeliverastatic\snapshot"oftheontologyintheform
ofanRDFStaxonomyhasprovedextremelyconvenientwhenworkingwith
collaboratorswhoarebuildingontologiesthatareinfactsimpletaxonomies,
suchastheGeneOntology [22].
5 Conclusion
Ontologiesareusefulinarangeofapplications,andwillplayapivotalr^oleinthe
SemanticWeb, wheretheywillprovideasourceofpreciselydenedtermsthat
canbecommunicatedacrosspeopleandapplications.Reasoningwithrespectto
suchtermswillbeimportantforboththedesignanddeploymentofontologies.
WehavepresentedOilEd,anontologyeditorthat hasaneasyto useframe
interface,yetatthesametimeallowsuserstoexploitthefullpowerofanexpres-
sivewebontologylanguage(OIL/DAML+OIL).WehavealsoshownhowOilEd
uses reasoning to support ontology design and maintenance, and presented a
casestudy illustratinghowthis facilitycan beused to developontologiesthat
describetheirdomainsin moredetailandwithgreateraccuracy.
OilEd is aprototype,designed to test and demonstrate novelideas, and it
stilllacksmanyfeaturesthatwouldberequiredofafully-edgedontologydevel-
opmentenvironment,e.g.,itprovidesnosupportforversioning,orforworking
withmultipleontologies.Moreover,thereasoningsupportprovidedbytheFaCT
systemisincompleteforOILextendedwithconcretedatatypesandindividuals,
anddoesnotinclude additionalservicessuchas explanation. However,in spite
ofthese shortcomings,OilEdisalreadysuÆcientlywelldeveloped tobeavery
usefultool,andto demonstratetheutilityofOIL'sintegrationoffeaturesfrom
frame,DLandweblanguages.
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