UMLR — UML 2.5 Overly strict restriction on message slope in seq diagrams

In UML 2.5 Paragraph 17.4.4. Notation

Message:
“..A line must be such that every line fragment is either horizontal or downwards when traversed from send event to receive event.”

While this restriction appears reasonable when first read, it is really overly strict.

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

In the default, WEAK, interpretation of sequence diagrams, the time-wise ordering of occurrences within a lifeline is independent of occurrences on other lifelines, subject to cause/effect sequencing (message sending—>message reception). And, of course, the order of occurrences as depicted on the lifeline must be maintained.

This is practically equivalent to saying that each lifeline has its own clock or timescale, and that ordering by that clock must be maintained and that causality across lifelines must be maintained. It is often thought that one could change the timescale (and not necessarily smoothly) on a lifeline without changing the interpretation or legality of the diagram.

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.

However, the restriction on messages not taking a non-negative slope, prevents otherwise legal changes in scale and makes invalid some diagrams that do not violate causality or the within-lifeline ordering.

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.