The items of clusters and structures have a cardinality of 1..* at the level of the reference model. I believe this should be broadened to 0..*. The reason is that there are inevitably some situations where it may be necessary to save a composition without elements in some structures. For example, a doctor adds a new cluster (through a GUI process) perhaps names it from a terminology and then is called away from the screen.
At some point this will be saved for update later. When so, it will need to be saved.
Although examples are a little contrived, one can also think of situations where a cluster might be added and left empty.
It just seems wrong, from a computer science point of view to disallow empty containers.