DateTime::Set is a module for datetime sets. It can be used to handle
two different types of sets:
* Fixed sets of predefined datetime objects. For example, if we wanted
to create a set of datetimes containing the birthdays of people in
our family.
* Recurring sets, such as "every Wednesday", or "noon on the 15th day
of every month". This type of set can have fixed starting and ending
datetimes, but neither is required.
