begin
abstract fun beginTravelTo(destination: LocationIfc, purpose: MovePurpose, waiter: ProcessModel.Entity): HoldQueue?(source)
Sends the vehicle to destination and reports where to wait for it.
Return
the queue to suspend waiter in, or null when the vehicle was already there
Parameters
purpose
what the journey is for. Never what the vehicle is carrying, which the substrate reads for itself
waiter
whoever is to be resumed when the journey ends -- the vehicle's own driver or agent, never a load being carried, whose waiting belongs to its own protocol