public void SetSystemReference(AlternativeSystem system)
 {
     this.SystemReferenceIdentifier = system.Identifier;
 }
 private ISet<AlternativeSystem> GetAllSubsystems(ISet<AlternativeSystem> systems, 
                                                   AlternativeSystem system)
 {
     systems.Add (system);
     foreach (var s in system.Alternatives) {
         GetAllSubsystems (systems, s);
     }
     return systems;
 }