/**
 * A calendar is composed of zero or more scheduled items.
 */
abstract class Calendar {
    Collection<ScheduledItem> items;
}

/**
 * A ScheduledItem is the generic definition for the types of items stored in a
 * calendar.  A ScheduledItem has a title, start date, and end date.
 * Extensions of ScheduledItem are Appointments, Meetings, Events, and Tasks.
 */
abstract class ScheduledItem {
    components: Title and StartDate and EndDate;
}