UML25 — Owning Rules

There should be a constraint that all Elements are owned, except the top element.
Also, the text refers to models and packages, yet these are not defined yet. Perhaps there should be forward reference to model, and the reference to package should be deleted.
Source: Sandy Friedenthal
Discussion:
Are root packages required to be "models"?

There is already a constraint that all elements must be owned, Element::has_owner (except of Packages, for
which the underlying mustBeOwned() operation is overridden.
The word “model” is used in 7.2.3 in a colloquial sense, not as a reference to the specific Model construct.
It is not the case that root Packages are required to be Models.
It is agreed, however, that it would be useful to include a forward reference to the Packages clause.