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Being able to place limits on Events that relate to other Events is a powerful feature. The Constraints element allows an Event to be sequenced relative to one-or-more other Events.





[ <AfterEvent Key=’key’/>  |  <BeforeEvent Key=’key’/> ]

 | <FromEvent Key=’key’/> ]  |  <UntilEvent Key=’key’/> ]

 | <AtEvent Key=’key’/> ]  ...




The interpretations of the elements are as follows:


  • AfterEvent. Current event > specified event
  • BeforeEvent. Current event < specified event
  • FromEvent. Current event >= specified event
  • UntilEvent. Current event <= specified event
  • AtEvent. Current event = specified event


The context of the Constraint, i.e. whether it appears in a <When> or <Until> element, determines whether it relates to the start or end date. If the specified Event is a simple Event then its only date is always applicable. Only one constraint for a given context may be established between two particular Events (e.g. no duplication for a start date), and they must not be self-referential.


Note that AtEvent is subtly different to an IsEventLnk in the associated <When> or <Until> element. AtEvent is merely equating the dates of two Events whereas the IsEventLnk is providing a hierarchical association between two related Events.