A CalendarRule instance contains the data and algorithmic rule necessary to compute the date when a holiday will fall in any appropriate year.
 /// <summary>
 /// Adds a <see cref="CalendarRule"/> instance to the set maintained by the
 /// current instance.
 /// </summary>
 /// <param name="rule">The <see cref="CalendarRule"/> to be added.</param>
 public void AddRule(CalendarRule rule)
 {
     rules.Add(rule);
 }
 /// <summary>
 /// Adds a <see cref="CalendarRule"/> instance to the set maintained by the
 /// current instance.
 /// </summary>
 /// <param name="rule">The <see cref="CalendarRule"/> to be added.</param>
 public void AddRule(CalendarRule rule)
 {
     rules.Add (rule);
 }