public ContactsIsOnTracker( ContactList contacts )
     : base(contacts)
 {
 }
 public ContactsMonitorTracker( ContactList contacts )
     : base(contacts)
 {
 }
 public ContactsTracker( ContactList contacts )
 {
     this.contacts = contacts;
     this.contacts.Users.CollectionChanged += Users_CollectionChanged;
 }
 public ContactsWatchTracker( ContactList contacts )
     : base(contacts)
 {
 }