UMLR — What is a DurationConstaint

I've read the description of firstEvent four times and I still do not understand it. Rewrite so there is a simple if/else that avoids the need to do detailed comparison between two very similar sentences.

Suggest:

A DurationConstraint is a Constraint that refers to a DurationInterval. which may be the interval between execution entering and exiting a single constrained element, or between selectable enter/exit events on a pair of constrained elements.

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When there are two constrainedElements, firstEvent[i] is true to select the enter, or false to select the exit event for execution of the corresponding constrainedElement.