Ok;
I have found one myself which has nothing to do with the things Adrian
told me...
Many->Many relations where the -> is modeled as a firstclass entity are
Many->One, One-> Many not contained in neither the left nor the right many.
example: Invocations are firstclass relations in Famix. The sender nor
the receiver are container, while it would be so if I would derive
container in the way I proposed.