awaited
The zone the transporter is waiting for, or null when it is not waiting.
This is the one thing ZoneHolderIfc asks of a holder beyond its name, because it is the queuing edge of the wait-for graph. A transporter is the holder that actually has a zone to report; a holder that only occupies space answers null, and the walk ends at one that is merely holding space. What a closure still waiting to drain is held up by is reached through its reservation instead, not through this property.