UMLR — Sequence Diagram: Message limitation

In several places in the UML 2.5 spec the message line (that goes from lifeline to lifeline) is restricted to have the destination end no higher than the source end.

While this sounds reasonable, it’s not really logically required in the general case.

No, I’m not talking about faster-than-light messages.

When a sequence diagram / fragment is “weak” (not strict) ordering, the individual life-lines can have their own time scale, and as long as causality is followed. And as each life-line’s time scale need not be uniform, a message going from one location on one lifeline to another lifeline but physically higher need not violate any logical or physical limitations.

Please eliminate the restriction preventing upward aiming message lines when the ordering is weak. This will allow the modelers to have better use of the diagram real-estate.

Michael