Go backwards to a previous point in the trajectory. Useful to implement loops.

rollback(.trj, amount, times = 1, check)

## Arguments

.trj
the trajectory object.
amount
the amount of activities (of the same or parent trajectories) to roll back.
times
the number of repetitions until an arrival may continue.
check
a callable object (a function) which 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.

## Value

Returns the trajectory object.