ReusebyContract Abt.Softwaretechnologie,TechnischeUniversitatBraunschweig GregorSnelting BerndFischer Bultenweg88,D-38106Braunschweig,Germany Email:ffisch,sneltingg@ips.cs.tu-bs.de Fax:+49-531-391-8140 Tel:+49-531-391-7579 nentsareassociatedwithcontracts formalmodelsoftheirfunctionalbehaviour andadmin- istered,retrieved,andreusedbythese.wearguethatreusebycontractisnecessaryforsafe solutionsinordertomakereusebycontractpractical. Keywords:formalmethods,reuse,softwarecomponentretrieval,specicationmatching,au- Reusebycontractistheapplicationofformalmethodstosoftwarereuse:softwarecompo- Abstract reuseinaformalprocessmodel,butishelpfulevenformoretraditionalsoftwaredevelopment. Wediscusssomeobstaclesagainsttheuseofformalcomponentspecications,andproposesome acceptanceproblems;discusstoolarchitectures;studyrelationwithoodesignmethods;study tomateddeduction. WorkshopGoals:Discussthedesignofreusablecomponentlibraries;discusswaystomitigate combinationwithstructuralcodereusemechanism(e.g.,patternsorfunctors.) 1

areassociatedwithcontracts formalmodelsoftheirfunctionalbehaviour andadministered, Reusebycontractistheapplicationofformalmethodstosoftwarereuse:softwarecomponents 1 Background retrieved,andreusedbythese. Similarapproacheshavebeenproposedbefore(e.g.,[KRT87,RW91,MM91])butwithoutconvincingsuccess.ThegoalofourprojectNORA/HAMMR1[FKS95c,FKS95b,FKS95a]isthustomake reusebycontractpractical.weinvestigate reuse-friendlyspecicationtechniques, libraryorganizationtechniquesbasedoncontracts, scalableandecientarchitecturesforreusebycontract, theintegrationofreusebycontractandconventionalreusemechanisms. itsinteractionwithformalsoftwaredevelopmentprocesses,and deductivecomponentretrieval, Ourlong-termgoalistobuildasystemwhichsmoothlyintegratescomponentdesign,implementation,andvericationwiththesystematicreuseofafullyspeciedandveriedcomponentlibrary. ofautomateddeductiontechniquestosolvetheprooftasksemergingfromdeductivecomponent OurworkonNORA/HAMMRstartedin1994.Ourmaintopicsofarhasbeentheapplication jointresearchprojectondeductionshowthatcurrenttheoremproversarecapabletosolveenough retrieval.alargenumberofexperimentsdoneincollaborationwithcolleaguesfromthegerman onanimprovedlibraryorganziationandtheintegrationofaprogramvericationsystem. contractlanguagebutcangenerateprooftasksfordierenttheoremprovers.wecurrentlywork oftheemergingtasks.ourimplementationistailoredtowardstheseexperiments.itusesvdmas 22.1WhatisReusebyContractreally? Position intotheactualmediumbywhichclientandprovidercannegotiatereuse: whichemphasisestheimportanceofformalspecicationsandinterleavesthemwithactualcode. Reusebycontractisanattempttoliftthisstyletocodereuse.Itsbasicideaistoturnthecontracts B.Meyerhascoinedthephrasedesignbycontract[Mey92]todenoteasoftwaredevelopmentstyle sponsoredbythedfgundergrantsn11/2-3. 1NORAisnorealacronym;HAMMRisthehighly-adaptivemulti-methodretrievaltool.Thisworkhasbeen Theclientstateshissideofthecontractasapairofgrantedpre-andrequiredpostcondition. 2

Negotiationisdenedintermsofthecontract:anoeredcomponentissuitedforreuseif Theproviderformalizeshisbid(i.e.,eachlibrarycomponent)theotherwayround,asapair theinvolvedpre-andpostconditionssatisfyawell-denedlogicalrelation. ofrequiredpre-andgrantedpostcondition. problemtobesolvedandassuchsometimesidentiedwithreusebycontract.inouropinion, reuseeectsandthusjustifythename\reusebycontract."inaformalsetting,thecontracts Thenegotiationprocess,alsocalleddeductivecomponentretrieval,isthemostimportanttechnical however,onlyitsintegrationintoaformalsoftwaredevelopmentprocesswillleadtosignicant arisenaturally(e.g.,fromrenements)anddonotimposeanyextraworkonthedevelopers. Ontheotherside,designbycontractisnotpersereusebycontractastheexistenceofalibrary doesnotautomaticallyimplyitsre-use.thedierenceisintherolesthecontractsplay.inthe Consequently,reusecanbebuiltintoprogramdesignfromtheverybeginning. designapproach,contractsarepassiveandconnedtothelibrary.theydonotonlydescribethe componentspropertiesbutarealsotheirpossessions.thereuseapproachremovesthisasymmetry contracts. and\activates"thecontractsforprospectiveclients.itisawaytoexploitthefullpowerof Prooftasksformalizethisrelation;theirexactnaturedeterminesformandeectsofreuse. Componentscanbereusediftheybridgethegapbetweentheclientsstatedpre-andpostcondition. 2.2ProofTasksandCodeReuse andhiswishes(thepostcondition)isbridgedcompletely.acomponentccanbepluggedinand hisqueryq.thisidealsituationisusuallyformalizedinthefollowingcondition,orprooftask:2 thusclosethegapifithasaweakerpre-andastrongerpostconditionthantheclientrequiresin Themostecientformofreusetakesplaceifthegapbetweentheclient'soer(theprecondition) qevenifitsresultsontheextendeddomaindonotttheoriginalquery,wemustrestrictthe Thisprooftaskishowevernotadequateifqisapartialfunctionbutnotc.Ifwewantctomatch (preq)prec)^(postc)postq) (1) tasksoftheform Thisso-calledplug-incompatibilitysupportssafereuse.Theretrievedcomponentsmaybeconsideredasblackboxesandmaybereused\asis",withoutfurtherprovisoormodication. (preq)prec)^(preq^postc)postq) (2) implicationbetweenthepostconditionsonthedomaingivenbypreq.wethusworkwithproof Another,weakerformoftheprooftasksemphasizestheclientspostconditionandretrievesall componentswhichsatisfyitatleastontheirowndomain: logicalfunctionsoftherespectiveparameters.however,toimprovethelegibility,weusethistraditionallyabbreviated andthequeryandalsocontainequationsrelatingtheparameters.likewise,thepre-andpostconditionsareofcourse 2Actually,theprooftasksareuniversallyclosedwrt.theformalinputandoutputparametersofthecomponent prec^postc)postq (3) formulations. 3

hastosatisfytheopenobligationprec.inaformalframework,however,(3)isjustiedbecauseit describesanormalrenementstep:theclienttradeshisownopencontractpostqagainsttheusually Generally,codereusebasedonthisconditionalcompatibilityispotentiallyunsafebecausetheclient tobeproven. Toincreasetherecall,conditionalcompatibilitycanberelaxedfurtherbyagaintakingtheclient's simplerprecbyre-usingc.notethat(3)isalsomoreecienttocheck,asonlyoneimplicationhas preconditionintoaccount.butincontrastto(2)itisnowaddedtothepremise: restrictedbyprec^preq.byvaryingpreq,clientscancontrolrecallandgranularityofreuse.the Hence,partialcompatibilityretrievesallcomponentswhichdo\therightthing"atleastonadomain prec^preq^postc)postq (4) strongeritis,themorecomponentsareretrievedbutthesmallleristheirrespectivebenet,simply 2.3Signicance becausepreqactsasanadditionalopenobligation. Theinvestigation[Lio96]oftherecentAriane5disasterrevealedthatitwascausedbythereuse ofanunmodiedariane4softwarecomponentwhichledtoanuncaughtexceptioncrashingthe reasonforthecrashwasthecomponentsfailuretostateitsassumptions,i.e.,theabsenceofa softwareandhencethespacecraft.in[jm97],however,jezequelandmeyerarguethattheultimate contract.theyconclude \Thereisamoresimplelessontobelearnedfromthisunfortunateevent:Reusewithout acontractisasheerfolly.fromcorbatoc++tovisualbasictoactivextojava, ofpotentiallydisastrousconsequences." process.toattempttoreusesoftwarewithouteiel-likeassertionsistoinvitefailures hopesaredoomedtoproduceresultsfarworsethanatraditional,reuse-lesssoftware thehypeisonsoftwarecomponents.theariane5blundershowsclearlythatnave Wesharethisconclusionasmotivationforourwork. 2.4Benets Fortheusers,thebiggestvisiblebenetisofcoursetheabilitytoretrievecomponentswhich provablymatchtheirneeds.provablymatchingcomponentsincreasetheoverallqualityofthe benetscanbeidentied. software.theyalsoimprovethesoftwareprocess,theproductivityandotheraspectsofsoftware Plug-incompatibility{and,toasmallerextent,conditionalcompatibility{aremostusefulin development.dependingonthekindofcompatibilityused(plug-in,conditionalorpartial),several aformaldevelopmentprocess.here,itcanbeusedforthesafecompositionofcomponents.a componentwhichsatisesaprooftaskisguaranteednottocompromisetheoverallcorrectness. Thisistrueevenforconditionalcompatibility,if{asisoftenthecase{therenementprocesscan generatethecomponent'sprecondition.safecompositionpreventsreusedisastersliketheariane 4

Insomecasessoftwarecompositionfromformallyspeciedcomponentscanbedoneautomatically. componentreuse,andreusebycontractistheonlyavailabletechnology. case.safecompositionisamustforanysafety-criticalsoftwareprojectwhichwantstoutilize Bymeansofconstructivetypetheory,aformalspecicationcanbetransformedintoexecutable codewhichmayalsocontaincallsto(formallyspecied)librarycomponents.needlesstosay,the resultingcodeisprovablycorrect. Onceanewpieceofsoftwarehasbeenconstructedbysafecomposition,automaticallyornot, Developmentstepsarelarger,andanactualimplementationforsubsystemscanbeobtainedearly, anyway.furthermore,reusebycontractoersadditionalsupportforaformaldevelopmentprocess. overhead:inaformalprocess,thespecicationofanewcomponentorsubsystemmustbesupplied itcanbeaddedtothelibrary.hencethebasisforreuseisincreased,withoutanyadditional feedbackisobtainedwhichencouragestheuseofformalspecicationsanddedicatedtechnology therebysupportingverticalprototyping.ifnewcomponentsareaddedtothelibrary,apositive andtools(e.g.veriers)foraformaldevelopmentprocess. requiremanualchecksorevencomponentmodicationsinordertomatchthecomponentsprecondition.forasoftwaredeveloperwhoseprimaryinterestistondreusablecomponents,thisisnot ordertoutilizepartialcompatibility.still,useofaformalpostconditionconsiderablyincreasesthe precisionofcomponentsearch,thatis,reducestheprobabilityofndingirrelevantcomponents. anobstacle.iftheprimarygoalistoreusecode,thedeveloperwillbereadytoprovidethestronger context,partialcompatibilitywillbemostuseful.partialcompatibilityhashighestrecall,butmay Inalessformalsoftwareprocess,reusebycontractstillimprovessoftwaredevelopment.Insucha preconditionneededforconditionalcompatibility,orevenextendthecomponent'sfunctionalityin notformallyspecied.ifdierentvendorsoerthesamelibrary,therewillbesubtledierences Reusebycontractisattractiveevenforthosewhoprefercommercialsuccessoversafetyorimproved componentretrieval.today'slibraries(forexample,thestandardtemplatelibraryforc++)are betweenimplementations.asaconsequence,auserwillnotbeabletoswitchfromonelibrary animplementation-independentspecication{astandard{foracomponentlibrary.today,such standardshavebeenestablishedinothersoftwareandhardwareareas;usageofformalspecications implementationtoanother{enablinglibrarymonopoliesandpreventinginnovativevendorswith smallmarketsharefromcommercialsuccess.hencemarkettransparencyrequiresthatthereis 2.5Obstacles asstandardsforlibrarieswillresultinbetterreturnoninvestmentforindependentlibraryvendors. colleagueputit:\ifineedasortroutine,isaygrepsort!" hardtask.themajorimpedimentisthegeneralacceptanceproblemofformalmethods.asone Thereareacoupleofproblemswhichmakeasuccessfulimplementationofreusebycontracta Withoutaformalsoftwaredevelopmentprocess,theup-frontcostsbecomefairlyhigh.Programmersarenotusedtocontractingandmayconsideritmerelyasanadditionalburdenwhichremains thefeed-backdescribedabove.worstofall,duetothegeneralindierenceinformalmethods,the construction,however,istime-consumingandexpensive,especiallywhenitisnotsupportedby withoutanybenetaslongastherearenoornotenoughspeciedlibrariestobereused.library marketoersonlyveryfewspeciedlibrariestobeginthisprocesswith. 5

complicatedparametersettingsfortheprover,specifydetailsheconsidersirrelevant,orprovide postconditionsinsomecrypticproverlanguage,reusebycontractwillnotbesuccessful. The\lookandfeel"ofareusesystemalsocanimpairitsusefulness.Iftheenduserhastodealwith Anothersourceofproblemsisthecomputationalcomplexityofdeductivecomponentretrieval.Long moretechnicalaspectmayleadtoevenmorecomplicatedprooftasks.ifproviderandclientuse componentswhichinturnpromiselargerpay-osmaybecometoolargeortoocomplicated.a responsetimesduetoinsucientdeductivepowercaneasilyrendertheentireconceptimpractical; dierentmathematicalconcepts(e.g.,setsandlists)theresulting\viewmismatch"canonlybe thisinparticularaectsscale-upforbiglibrariesorcomplicatedcomponents.contractsforlarger Inanon-formalsoftwareprocess,theuseofformalspecicationscanevenhampertheretrieval solvediftheproverdeducesthenecessarymappings. abilities.thereasonisthatrecallmaysuerfromtheoverwhelmingprecisionofformalspecications.ifthereisacomponentwhichdiersonlyslightlyfromwhattheuserwants,itwillnot falsestatementswhichmaybackreinareusecontext.notethatinaformalprocess,thisproblem doesnotoccuraspartialor\fuzzy"contractfulllmentisnotacceptable. befound,becausetheproofobligationcannotbefullled(\nearmiss").theoremproversdonot haveanotionofan\almostprovable"statement,anditisthissharpdistinctionbetweentrueand Thereisnomedicineforpeoplewhorejectformalmethods.Theonlyargumentwhichmightappeal 2.6PossibleSolutions Firstofall,aretrievalsystembasedonformalspecicationsmusthidethedeductivemachinery buttoaugmentthem.foralltheotherproblems,solutionscanbeoutlinedasfollows. tothemisthatreusebycontractisnotgoingtoreplaceexisting,establishedretrievalmethods, eratingproverinputetc.mustbeinvisibletotheuser.instead,theretrievaltoolshouldoer completely.anydetailsforsettingproverparameters,synchronizingparallel-runningprovers,gen- expert.theendusermustnotbebotheredbythis. accesstomoretraditionalretrievalalgorithms.assomebodyhastotakecareofpreprocessingand aninterfacewhichutilizestheenduser'slanguageandconcepts;inparticular,itshouldalsooer Inordertotackletheperformanceandscale-upproblems,weutilizetwomechanisms:abstraction tuningtheformalspecicationsandtunethedeductiveengine,weproposethatthisisdonebyan needed,somtimesamorecompactcomponentdescriptionissucient.multiplelayersofspecicationscanbeusedtoseparatethecorefunctionalityfromnon-functionalimplementationaspects ase.g.structuresharing[ph95].thiscanbeachievedbyadomain-speciclogicwhichwouldnot Incrementalitymeansthatseveralprocessesmustcooperateinordertoachieveanincreasingreductionoftheproblemspace.NORA/HAMMRusesalterchaininordertoreducetheburden onlyimprovethedeductiveabilitesofthesystembutwouldalsobebenecialfortheenduser.3 andincrementality.abstractionmeansthatnotalwaysatraditionalformalspecicationwillbe ofthetheoremprover.thechainconsistsofaseriesofltersofincreasingpower,theproveris setofpredenedfunctionandpredicatenamesisdened,whosemeaningisgivenbysomeadditionalaxioms. signaturematchingandmodelchecking.signaturematchingselectscomponentsaccordingtoa onlythelastelementinthechain.chaincongurationmayvary;atypicallterchainincludes 3Itistechnicallyeasytocreateadomain-specicextensionofaspecicationlanguage:thisonlyrequiresthata 6

specicationoftheirinterfacealone.modelcheckingisusedinordertodiscovernon-theorems:ifa counterexampleinsomesmallmodelcanbefound,theproofobligationisconsideredanon-theorem andthecomponentrejected.bothtechniquesmaynegativelyaectrecallaswellasprecision,as numberofproofobligationssurvivesthepreliminarylters. interfacesistoorestrictive.4buttheygreatlyreducetheburdenoftheprover,asonlyasmall acounterexampleinanitemodelof,say,theintegersmaybeinvalid,anddemandingidentical similaritiesbetweenthetypesinquestionwhichinturncanbeusedtoconstructsomeofthe abstractionfunctionsautomatically. Signaturematchingcanalsohelpwiththeviewmismatchproblem.Itidentiesthestructural oftheprover,weagainproposeabstractionandincrementality.theuserinterfacemustallowto ingthesetofsurvivingcomponents.furthermore,useofadomain-specicspecicationlanguage incrementallysharpenthepostcondition(orweakentheprecondition),therebyincrementallylter- Inordertoreducetheriscofnotndingcomponentsduetooverspecicationor\nearmisses" oerstheappropriateabstractionstotheuser;nearmissesduetoerroneouslow-levelspecication detailsareavoided. descriptionsandreferenceimplementations.theindustrial-strengthexampleofacontract-based 3Mostworkonlibrarydesign(e.g.,[MS96,Knu93])followsthetraditionalstyleofinformalorstylized Comparison asubsumptionrelationbetweencomponentsandstructuretheirlibrariesaccordingly.similarly, librarydesignweknowofismeyer'swork[mey94].thereis,however,somemoreresearchwork,e.g. [LW94]denethenotionofbehavioralsubtypeasanextendedmeanstoorganizeclasslibrariesand theresolveproject[sw94].both[jc93]and[mmm94]useformalspecicationstodetermine [Lea91]hasdevelopedtechniquestosupportthespecicationandvericationofobject-oriented thecomponentsanditsbuilt-inhigher-orderunicationasretrievalmechanism.moormanzaremski programs. andwing[mw95]werethersttoexploredierentmatchrelations;ourownwork(cf.section2.2) expandsontheirresults.theyalsointroducedtheuseofa\real"specicationlanguage(larch/ml) Deductivecomponentretrievalhasalsobeeninvestigatedby[RW91]whichusedPrologtospecify forcomponentdescription.withtheexceptionof[ste91]whichworkswithalgebraicspecications, mostotherapproachesnowalsouselanguageswhicharesomesugaredvariantofrstorderlogic. However,while[MW95]appliestheassociatedinteractiveLarchProvertosolvetheprooftasks, Amorepragmaticapproachtodeductiveretrievalistousethecomponentstypesastheirspecications(e.g.,[Rit91].)Thissignaturematchingallowstheapplicationofmoreecientreasoning isthusasuccessfultoolforfunctionallanguageswhichoerrichtypestructures.thebehavioral bycontractbutitcanstillbeusedasafastpre-lter. mechanisms(e.g.,order-sortedtheoryunication)butalsoaveryconcisequeryformulation.it [FS97]tomakeitpractical. theirsheernumberrequiresanautomatedtheoremproverase.g.,otter[mmm94]orsetheo abstractionwhichisinherenttotypesmakesanunmodiedtype-basedretrievalunsuitedforreuse andfortheformerweproposetheuseofabstractmodelchecking. 4Infact,thelatterproblemcanbetackledbyuseofadditionalaxiomswhichallowe.g.interchangesofparameters, 7

environments.theparissystem[krt87]supportedthesemi-automaticconstructionofprograms overalibraryofso-calledschemes,i.e.,programfragmentswhichareenrichedbyassertionsabout theircombinationandinstantiationpossibilites.theconstructionprocessthengeneratedproof Therehasalsobeensomeearlierworktointegratedeductiveretrievalintosoftwaredevelopment taskswhichweresolvedbytheboyer-mooretheoremprover.theinscapesystem[per87]aimed andneverlefttheprototypestage. support.however,bothsystemsworkedwithseverelyrestrictedlogicsandinferencemechanisms atthedevelopmentoflargesoftwaresystemsbasedonspecications;italsooeredsomeretrieval Lowryetal.[LP+94]utilizedaformallyspeciedlibrarywhichcontainsfunctionsforcelestial mechanicsandspaceshipcoursecomputations.afterprovidingaformalspecicationofe.g.aspace vehicle'sdestinationpointandtime,thesystemautomaticallycomposesaprogramconsistingof callstoappropriatelibraryroutines,whichcomputetheightdata.lowry'ssystemusesadomainspeciclogic,whichinturnishiddenfromtheuserbyasophisticatedgraphicaluserinterface. Althoughspeciedcomponentlibrariesareanecessaryrequirementforanycodereusemechanism 4workingwithformalmethodsandnotonlyforreusebycontract,theyareanearlyextinctspecies. ResearchTopics speciedlibrarieswhichmustalsocovernon-functionalaspectsofcomponentswhichtraditionally WethinkthatthecommunityshouldtakeuptheleadofMeyerandworktowardsrealistic,formally matterfortheusers,e.g.structuresharing.thisworkshouldalsoincludethedevelopmentof appropriatedomain-speciclogics. nentsrequiresalotoffurtherresearch.firstofall,exiblecomponentsareparameterized(generic Largerbenetsareexpectedfromreusingcomponentsofamuchcoarsergranularitythansimple Anextstepcouldbeaninvestigationaboutspecicationsofdesignpatternsorparameterized packages,c++templatesetc).thusevensignaturematchingrequireshigher-orderunication. functions(\megaprogramming",[wwc92].)scaling-upspecicationmethodstosuchmegacompo- situationwheretherearethousandsofsimpleproofobligations,andalmostallofthemarenontheorems.furthermore,methodshavetobedevelopedforfastrejectionofnon-theoremswhichdo Scale-upalsoconcernsdeductiveretrieval.Firstofall,theproversmustbeadaptedtoreecta modules/functors.havingafullyspeciedpatternlibrarywouldbeaniceargument. notcompromiserecall.onepromisingcandidateisabstractmodelchecking[jac94].probablythe couldbyrelaxedbyintroducingsomeapproximatereasoning.however,thisrequiresappropriate Finally,toincreasethenumberofreuseopportunites,thestrictcompatibilitiesdenedin2.2 combinationofbehaviouralsubtypingandsignaturematchingalsoworkstothisend. automaticcomponentadaptionmechanismstomaintaintheintegrityofreusebycontract. References [FKS95a]B.Fischer,M.Kievernagel,andG.Snelting."Deduction-BasedSoftwareComponent Retrieval".InJ.Kohler,F.Giunchiglia,C.Green,andC.Walther,(eds.),Working 8

[FKS95b]B.Fischer,M.Kievernagel,andW.Struckmann."High-precisionretrievalforhighqualitysoftware".InI.M.Marshall,W.B.Samson,andD.G.Edgar-Nevill,(eds.), andprograms,pp.1{5,montreal,august1995. NotesoftheIJCAI-95Workshop:FormalApproachestotheReuseofPlans,Proofs, [FKS95c]B.Fischer,M.Kievernagel,andW.Struckmann."VCR:AVDM-basedSoftwareComponentRetrievalTool".InM.Wirsing,(ed.),WorkingNotesoftheICSE-17Workshop Dundee. Proc.4thSoftwareQualityConf.,pp.80{88,Dundee,July1995.UniversityofAbertay [FS97] onformalmethodsapplicationinsoftwareengineeringpractice,pp.30{38,seattle, B.FischerandJ.M.P.Schumann."SETHEOGoesSoftwareEngineering:Application Wash.,April1995. [Jac94] D.Jackson."AbstractModelCheckingofInniteSpecications".InM.Naftalin, ofatptosoftwarereuse".inw.mccune,(ed.),proc.14thcade,lnai1249, T.Denvir,andM.Bertran,(eds.),Proc.2ndFME,LNCS873,pp.519{531,Barcelona, Townsville,July1997.Springer. [JC93] J.JengandB.H.C.Cheng."Usingformalmethodstoconstructasoftwarecomponent October1994.Springer. [JM97] J.-M.JezequelandB.Meyer."DesignbyContract:TheLessonsofAriane".IEEE library".ini.sommervilleandm.paul,(eds.),proc.4thesec,lncs717,pp.397{ Computer,30(1):129{130,January1997. 417,Garmisch-Partenkirchen,September1993.Springer. [Knu93] [KRT87]S.Katz,C.A.Richter,andK.S.The."PARIS:ASystemforReusingPartiallyInterpretedSchemas".InProc.9thICSE,pp.377{385,Montery,CA,March1987.IEEE D.E.Knuth.TheStanfordGraphBase:APlatformforCombinatorialComputing.ACM Press,NewYork,1993. [Lea91] G.T.Leavens."ModularSpecicationandVericationofObject-OrientedPrograms". IEEESoftware,8(4):72{80,July1991. ComputerSocietyPress. [Lio96] [LP+94]M.Lowry,A.Philpot,T.Pressburger,andI.Underwood."AMPHION:automatic J.L.Lionset.al.Ariane5Flight501FailureReport,1996. (eds.),proc.8thintl.symp.onmethodologiesforintelligentsystems,lnai869,pp. programmingforscienticsubroutinelibraries".inz.w.rasandm.zemankova, [LW94] B.LiskovandJ.M.Wing."ABehavioralNotionofSubtyping".ACMTOPLAS, 326{335.Springer,October1994. [Mey92] B.Meyer."Applying\DesignbyContract"".IEEEComputer,25(10):40{51,October 1992. 16(6):1811{1841,November1994. [Mey94] B.Meyer.ReusableSoftware:TheBaseObject-OrientedComponentLibraries.Prentice- Hall,EnglewoodClis,1994. 9

[MMM94]A.Mili,R.Mili,andR.Mittermeir."StoringandRetrievingSoftwareComponents:A [MM91] P.ManhartandS.Meggendorfer."Aknowledgeanddeductionbasedsoftwareretrieval tool".inproc.4thintl.symp.onarticialintelligence,pp.29{36,1991. [MS96] D.R.MusserandA.Saini.STLTutorialandReferenceGuide.Addison-Wesley,1996. Italy,May1994.IEEEComputerSocietyPress. Renement-BasedSystem".InB.Fadini,(ed.),Proc.16thICSE,pp.91{102,Sorrento, [MW95]A.MoormanZaremskiandJ.M.Wing."SpecicationMatchingofSoftwareComponents".InG.E.Kaiser,(ed.),Proc.3rdFSE,pp.6{17,Washington,DC,October1995. [Per87] ACMPress. D.E.Perry."TheInscapeEnvironment".InProc.11thICSE,pp.2{12.IEEEComputer [PH95] A.Poetzsch-Heter."InterfaceSpecicationforProgramModulesSupportingSelective UpdatesandSharingandtheirUseinCorrectnessProofs".Softwaretechnik-Trends, SocietyPress,May1987. [Rit91] M.Rittri."Usingtypesassearchkeysinfunctionlibraries".JFP,1(1):71{89,January 15(3):116{125,October1995.Proc.Softwaretechnik95,G.Snelting(ed.). [RW91] E.J.RollinsandJ.M.Wing."SpecicationsasSearchKeysforSoftwareLibraries".In 1991. [Ste91] R.A.Steigerwald.ReusableSoftwareComponentRetrievalviaNormalizedAlgebraic K.Furukawa,(ed.),Proc.8thIntl.Conf.Symp.LogicProgramming,pp.173{187,Paris, June24-281991.MITPress. [SW94] M.SitaramanandB.W.Weide."SpecialFeature:Component-BasedSoftwareUsing RESOLVE".ACMSIGSOFTSoftwareEngineeringNotes,19(4):21{22,October1994. Specications.PhDthesis,NavalPostgraduateSchool,December1991. [WWC92]G.Wiederhold,P.Wegner,andS.Ceri."Towardmegaprogramming".Communications Biographies oftheacm,35(11):89{99,november1992. ofbraunschweig.hisinterestsincludeformalspecication,automateddeductionandfunctional programming.hereceivedhisdiplomaincomputersciencefromthetubraunschweigin1990. BerndFischerisresearcherattheDepartmentofSoftwareTechnologyattheTechnicalUniversity schweig.hismaininterestistoutilizedeductiveandalgebraictechniquesinordertoimprove softwaredesign,congurationmanagement,componentreuse,softwarereengineering,andsoftware GregorSneltingisaprofessorforsoftwaretechnologyattheTechnicalUniversityofBraun- validation.hereceivedadiplomaincomputerscienceandmathematics(1982)andaphdin technologygroupin1991. computerscience(1986)fromthetudarmstadt,andbecameprofessorandleaderofthesoftware 10