UMLR — Owning of interaction fragments is ambiguous when InteractionOperands are present

It is not clear from chapter 14 how interaction fragments are supposed to be owned when there are InteractionOperands present.

It seems to be the case, but is not stated, that everything diagrammatically inside the operand should be owned by the fragment. This would, I think, give rise to the following consequences:

1. The top and bottom of each fragment and operand must be on the same Lifeline or Execution. A fragment cannot span different executions or have its boundaries cover different levels of execution nesting.

2. Everything inside of a fragment/operand must be entirely contained by the fragment/operand. This includes both sides of a message, all nested fragments, interaction uses, and the top and bottom of execution specifications.

However it appears to be a valid instance of the metamodel to parent arbitrary fragments at any level of nesting, which would enable these constraints to be violated.

The specification should confirm these ownership constraints. This would best be done in conjunction with the sentence “InteractionOperand contains an ordered set of InteractionFragments” in section 14.3.16, which should state exactly which InteractionFragments must be owned by the InteractionOperand.

Different vendors’ interpretations of this ambiguity can cause interoperability problems.