In some corner cases a background time slot can be completely
replaced by forground time slots. For example in this sequence
off peak -> peak -> mid peak -> peak -> off peak
If the mid peak slot is added after the peak slots the background
time slot covering that period needs to be removed. The previous
version left a zero length background slot that caused the Tasla
API to reject the tariff.
In some corner cases a background time slot can be completely replaced by forground time slots. For example in this sequence
off peak -> peak -> mid peak -> peak -> off peak
If the mid peak slot is added after the peak slots the background time slot covering that period needs to be removed. The previous version left a zero length background slot that caused the Tasla API to reject the tariff.