A set of assertions about the provenance of things
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Observed Time allow for interval of observation,
where an event is said to occur no earlier than a given time t1 and
no later than a given time t2. When the event is observed to occur
at a specific time, it is not convenient to use an interval.
Instead, one can use the alternate exactlyAt attribute. We note that
exactlyAt is disjoint from noEarlierThan and noLaterThan.