For certain subjects you can specify that the double lessons cannot span over long breaks. For example you do not want to let children have their snacks in the middle of chemistry laboratory lesson.
First you have to add the breaks into the timetable:
Then by default all double lessons cannot span over these breaks.
If you allow certain double lessons to span while others not that you can define for each subject individually if double lessons of this subject can or cannot span over the breaks. You can do this in subject's Constraints:
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