UMLR — UML2.2. Contradications in 14.3.10

14.3.10: says “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.” However slightly lower down it says “The trace semantics of Interactions merely see an Execution as the trace <start, finish>. There may be occurrences between these. 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).” These appear to be directly contradictory.

Is it necessary for the start and finish occurrences of an ExecutionSpecification to be ExecutionOccurrenceSpecifications? Is it valid to have a MessageOccurrenceSpecification at the start and finish of an ExecutionSpecification? Is it valid to have both a MessageOccurrenceSpecification and an ExecutionOccurrenceSpecification representing the start/end of a ExecutionSpecification? Are Message reception and Execution commencement the same or different events?

Also the multiplicity on the source (non-navigable) end of ExecutionOccurrenceSpecification.Execution is 1, which makes the model clearly invalid. I believe it should either be 2 or 0..2, depending on the answers to the questions above.