inv startEndConstraint: self.stopTime > self.startTime
inv nonEmptyDescription: self.description != ""
inv positiveDuration: self.duration > 0
context DriveTrainSetup::duration():Integer post: result = self.stopTime – self.startTime

//context Activity::complete(d:Integer) pre: self.isConfirmed = false
//inv: self.moderator.gender = Gender::female
//inv: self->collect(participants)->size()>=2
//context Meeting inv: self.participants->size()>=2