UML23 — Ordered derived unions

When you specify a property as an ordered derived union, with the implied derivation of union-ing the subsetting properties, how do you specify the order of the resulting union?

This problem actually occurs in the UML spec itself, in the definition of ExceptionHandler::output_pins
which relies on the ordering of Action::output, which is a derived union.
We can specify a determinate order for the common case where all of the subsetting properties are class
owned and there is single inheritance: which is the case for Action::output. In other cases, we can just say
that the order is undefined.