days2tupl() returns a list associating each day with the set of tuples assigned to that day (how it will be necessary to allocate the day's lessons to the hours 1:7 so that the day's tuples fall at the same time of day).
The lesson schedule by day returned by mount_days() has the prof field of type 'character' (not 'factor'), how 'hours2lessons::mount_hours()' requires you to assign a day's lessons to hours 1:7.
Added daily_cls2prof() which lists by day the classes allocated to a given teacher.