I was permitted a very minor divergence from the no-backtracking rule on my recent eastbound round-the-world trip. Because of peculiarities in AF's schedules I had to fly JFK-CDG-Rabat and Marrakech-CDG-Berlin, even though RAK is about 1 degree 18' west of RBA. The rules exist, but the DL round-the-world staff aren't inflexible. In fact they're great for helping arrange an itinerary.