public Closure Clear(PpEntryEffect effect)
 {
     if (_Transitions == AvailableSlideEntryEffects)
         _Transitions = new List<PpEntryEffect>();
     else
         _Transitions.Clear();
     return this;
 }
 public Closure Remove(PpEntryEffect effect)
 {
     EnsureLocalCopy();
     _Transitions.Remove(effect);
     return this;
 }
 public Closure Add(PpEntryEffect effect)
 {
     EnsureLocalCopy();
     _Transitions.Add(effect);
     return this;
 }