EDIT: please ignore. I did not look well enough in the app menu.
After a challenge is finished, it disappears completely, i.e., there is no way we can ride it again. With continuous riding now a reality, it would sense to change that policy. Some of the challenges (like Mallorca, 11 cities) are great for continuous riding, but for someone who finished them, they are no longer available. It would make sense to re-incarnate them as a playlist, or simply to allow a challenge to be finished multiple times.
