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Should the crossing be held shut against vehicles?
Asked whenever the crossing's state changes. True takes the zones -- draining whatever traffic is on them first, through the ordinary all-or-nothing machinery -- and keeps them until this answers false and the last pedestrian is off.
Answering true with nobody waiting shuts the crossing for no reason; answering false while people are on it does not evict them, because a turn always finishes.
Parameters
crossing
the crossing being asked