Undo set

The problem I have encountered - since there still isn't support for an Apple Watch app and the screen needs to remain on throughout the workout to maintain progress - is that the current set is marked completed while my phone is in my pocket. This triggers the timer and sometimes it also bumps the difficulty factor, which will affect my future workouts. There needs to be an undo button so that 1. the user is able to reset the timer and 2. the user can modify how difficult the first set was.