public void removeLesson(Lesson l)	{
		int i = this.timeTable.getSlots().indexOf(l);
		this.timeTable.getSlots().remove(i);
		this.timeTable.getSlots().add(i, l.getSlot());
		this.timeTable.checkForDuplicates();
		if (this instanceof Group)	{
			((Group)this).done.put(l.getField(), false);
		}
	}