From Andrew Patterson:
The HISTORY IM has a definition for a period which is meant to represent the time
between contained events. However, in the case where this period is filled in,
the spec makes no restriction on the EVENT.time field - i.e. what is the meaning
of a HISTORY object with a period set, but yet which contains EVENTs that are not
according to this periodic duration? It would be good if the spec
clarified what the intended behaviour of an openEHR system that encounters such
data should be.