UMLR — Chapter 14 is ambiguous and contradictory about how to link up messages and execution specifications

Chapter 14 is ambiguous and contradictory about how to link up messages and execution specifications. This is because in the metamodel the start and finish of an ExecutionSpecification are OccurrenceSpecifications, not ExecutionOccurrenceSpecifications. This means that it appears to be valid for the MessageOccurrenceSpecification that is a Message's receiveEvent to also be the start of an ExecutionSpecification.

The text is equally ambiguous. The 14.3.10 paragraph "An ExecutionSpecification is a specification of the execution of a unit of behavior or action within the Lifeline. The duration of an ExecutionSpecification is represented by two ExecutionOccurrenceSpecifications, the start ExecutionOccurrenceSpecification and the finish ExecutionOccurrenceSpecification" appears to say unambiguously that the start and finish must be ExecutionOccurrenceSpecifications. However the later sentence "Typically the start occurrence and the finish occurrence will represent OccurrenceSpecifications such as a receive OccurrenceSpecification (of a Message) and the send OccurrenceSpecification (of a reply Message)" both introduces ambiguity through the use of the word "typically", and then proceeds to blatantly contradict the earlier paragraph.

This causes tool interoperability problems.

I suggest targeting ExecutionSpecification::start and finish onto ExecutionOccurrenceSpecification, and rewriting the contradictory semantics accordingly.