Adds a new activity that goes backwards in the trajectory.
Usage
rollback(traj, amount, times = 1, check)
Arguments
traj
the trajectory object.
amount
the amount of activities to roll back (of the same level; it does
not go into branches).
times
the number of repetitions until an arrival may continue.
check
a callable object (a function) that must return a boolean. If
present, the times parameter is ignored, and the activity uses this
function to check whether the rollback must be done or not.