successors
The zones a transporter could enter in one step from this one, given the direction it faces.
From a link zone that is the direction of travel along the link, and from the last zone of a link the junction it leads to. From an intersection zone it is the first zone of every link that may be traversed away from that junction, including a bidirectional link approached from its far end.
Parameters
zone
the zone the transporter holds
travelling
the direction it faces on a link zone, ignored at an intersection