In a classic implementation of the Observer design pattern, the subject has a list of references to all of it's observers, and each observer might have a reference to it's subject.
Is this considered composition? As far as I understand, composition means one object 'includes' in itself another object, in order to use functionality of that object. In the Observer pattern, the subject does include in itself references to it's observers, but it doesn't exactly use their functionality internally to do things outwardly. It keeps references so it can notify them.
On the other hand, the subject and observers maintain a relationship of HAS-A, which often characterizes composition (the subject has observers, the observers have a subject).
Is this considered a kind of composition?