In the validty model, the class "TimePeriodOfDay" is an aggregation to the "Period" class.
The only possibility is to specialise this class by "TimePeriodOfDay", there are no other links or attributes.
I'm wondering, if this class cannot be removed to introduce a direct aggregation between "Period" and "TimePeriodByHour".
Agree it is currently useless, but it is the right name for the role of TimePeriodByHour, so the fix should probably be to move the attributes into TimePeriodOfDay, make that concrete, and remove the ByHour class.
Done in line with comment #1 above as agreed by TG. This meant that only one class was left in package TimePeriodOfDay, so moved the class to Validity and deleted the package TimePeriodOfDay.