invariant.st

   Object subclass: #Artist
     instanceVariableNames: 'tracks'
     methods for: 'initialising'
       initialize
         tracks := OrderedCollection new.
         ^self
     methods for: 'checking'
       invariant
         ^(tracks isMemberOf: OrderedCollection) and: [
           tracks allSatisfy: [:each | each isKindOf: Track]]
     methods for: 'accessing tracks'
       addTrack: aTrack.
         tracks add: aTrack. 
         ^track
       tracksDo: doBlock
         tracks do: doBlock