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sestchangestobeinterpretedandtheirinternalconsistencychecked.AppendixAalsoin-
cludessampleoutputshowingtheconsistencychecksandtheresultingaddressspacefora
fewsmall(butcomplex)instancesofPalimpsestchangesets.
Chapter5:ApplyingandEvaluatingtheModel
Inthischapter,IexaminetheimplicationsofthepropertiesofP-sequences,andtheir
applications.MuchofthischapterwillbeconcernedwithhowP-sequencescanbemanipu-
latedbyoperationsonchangesets.Beforeexaminingtheissuesindetail,itisworthre-
viewinghowthepuremodelinChapter4wouldbeusedinanactualsystem.Wehavede-
finedaP-sequenceintermsofasinglechangeset,andtherepresentationofasequence
stateorversionconsistsofachangeset.Relatedversionswouldbechangesetssharingmore
orfeweroperations.
Exceptinsomeofthefinaltheorems,wedidnotconsidermultipleP-sequencesinthe
previouschapter.Inasystemapplication,wewillalwaysbedealingwithP-sequenceswhose
changesaresubsetsofalargeruniversalsubset,representingallchangesevermadetoa
document.Individualcollaboratinginstancesofanapplicationwillmaintaintheirown“mas-
terchangesets”whichmaynotbecompletewithrespecttotheuniversalchangeset.In
termsofmodelinghowever,theuniversalsetsarenotimportant,becausetheimportant
questionforanapplicationiswhataparticularsetofchangesmeans:whatP-sequenceit
defines.ThesubjectsofChapter6andChapter7arethecreationofanefficientchangestore
forP-sequencesandaflexibleapplicationarchitectureforusingthatchangestore.
5.1 Editinghistoriesandversionmanagement
Inthissection,IwillbrieflydiscusssomeofthewaysthatPalimpsestcanmodelthefa-
cilitiesprovidedbymoretraditionalversionmanagementsystems,andexaminehowthe
commonestversioningmodelsfitintoPalimpsest’smodel.Thetraditionalapproachmanages
asetofdocumentstates(versions)bymaintainingaderivationgraph(see(Conradiand
Westfechtel1998)foranexcellentoverviewofthemanymodelsinthisarea).Thisversion
graphusuallytakestheformofaDAG,thoughisitsometimesfurtherrestrictedtobeatree.
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