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