QUEUE

Take a place in the queue behind whoever is already promised the zone.

The closure is accepted, waits its turn, and is granted when every zone of its set is both empty and promised to it rather than to somebody ahead of it. Order per zone is the order the requests were made.

Ask for this only when serialising the closures is what the model means. It is not a convenience for making an inconvenient refusal go away: two spills in the same aisle queued this way are cleaned twice, in series, which for one spill reported twice is simply wrong and is wrong quietly. Where it is right -- maintenance windows on one leg, a crossing served in turns, work that genuinely repeats -- it saves the model writing the waiting itself.