SYSML21 — Clarify that destroying a part doesn't destroy its subparts

Contrary to popular believe I don't think that destroying a Part destroys its subparts - according to the specification.

Let me explain:

7.9.1 Page 49: The extent of an occurrence in time is known as its lifetime,

That makes sense, but how many people are aware that not every occurrence is a Life? The lifetime of a portion might be much shorter than the lifetime of its Life. A note should clarify that.

7.9.1 Page 50: If an occurrence has values for any composite features at the end of its lifetime, then the lifetime of those composite values must also end.

Here the term "lifetime" suggests something, that is simply not true. And even the authors of the specification fall into this trap, as visible further down on the page:

7.9.1 Page 50: For example, if a wheel is attached to a car when the car is destroyed, then the wheel is also destroyed.

No, not necessarily. At least as far as I can see:

A composite part is variable in SysML. That means it is featured by snapshots of the whole. Since a whole usually has many snapshots and a part can also be a subpart of many wholes, there are quite a lot of snapshots featuring an occurrence of the composite part.

Now, composition also means exclusivity. Each occurrence can have at most 1 superoccurrence. And this is global. Out of all these snapshots of wholes at most one can reference the Life of the part. All the others must necessarily reference portions. I can see no reason, why the endshot of a specific whole should reference a Life.

Therefore the correct sentence would be:

If a wheel is attached to a car when the car is destroyed, then the portion of the wheel's Life, during which it is attached to a car, is ending.

This sounds much less dramatic. And is more like what we experience in real life.

There are two more places, where the destruction semantics should be clarified:

7.6.1 Page 32: if the containing instance is destroyed, then any instances related to it by composite usages are also destroyed. For example, a Vehicle would have a composite usage of its wheels...

7.6.3 Page 42: Values of a composite usage, for each featuring instance, cannot exist after the featuring instance ceases to exist.

Many people will not be aware that instances include portions.