/// <summary>
 /// Creates <see cref="UndoRedoHistory&lt;T&gt;"/> with given capacity.
 /// </summary>
 /// <param name="subject"></param>
 /// <param name="capacity"></param>
 public UndoRedoHistory(T subject, int capacity)
 {
     this.subject = subject;
     undoStack    = new RoundStack <IMemento <T> >(capacity);
     redoStack    = new RoundStack <IMemento <T> >(capacity);
 }