/// <summary>
 /// Removes the specified observe relation.
 /// </summary>
 public void RemoveObserveRelation(ObserveRelation relation)
 {
     _relations.Remove(relation);
 }
 /// <summary>
 /// Adds the specified observe relation.
 /// </summary>
 public void AddObserveRelation(ObserveRelation relation)
 {
     _relations.Add(relation);
 }