UML15 — UML2 super/Profile/Unconsistent Profile extension description

18.3.5 says that "A profile by definition extends a reference metamodel or
another profile."
While in theory I could see how it might be modeled, I don't see how the
latter could work in practice with real tools. Let's extend the current
example and define a new Profile called ClockTechnology with Stereotype
AtomicClock with baseClass Clock and property radioactiveElement:String..."
—

Import between profiles is supported, and stereotype generalization is the
usual way to achieve what has been called "extending a profile".

The reference to profile extension should be simply discarded. A profile
extends a reference metamodel.

see above