reviewAt

When the crossing should ask again even though nothing has happened.

A discipline whose answer depends on elapsed time needs this, or its answer never changes. The crossing asks its two questions when something happens -- somebody arrives, somebody leaves, a turn is granted -- and a rule like "open once the first of them has waited five minutes" turns on an instant at which, by construction, nothing happens. One walker waiting alone would wait for ever: nothing else is coming to prompt the question.

Return the instant to ask at, or NaN for "nothing is pending". The crossing keeps at most one review outstanding and re-reads this every time it asks, so a rule may move its own deadline freely.