prev-wp
Shift a waypoint reference to a previous waypoint in subslide order.
Given a waypoint label, returns a reference to the waypoint amount steps
before it. prev-wp(<c>, amount: 2) is equivalent to
prev-wp(prev-wp(<c>)).
When applied to a from-wp or until-wp marker, the shift is pushed inward:
prev-wp(from-wp(<c>)) becomes from-wp(prev-wp(<c>)).
Parameters
wp : label | dictionary
A waypoint label or marker to shift.
amount : int default: 1
How many waypoints to step back. Default is 1.
Returns: dictionary