CSDL for Collection types is not clear on nullability of elements

The CSDL spec for edm.CollectionType states:

"The collection type can define relevant facets for scalar types."

but does not specify what the relevant facets are. Also, it implies that a collection of collections cannot define facets (e.g. Nullable) for the element collections.

Also note thet edm.TypeDefinition does not permit the use of the Nullable facet, so an element type defined by a TypeDefinition cannot help resolve the issue.

So some definite questions:

(1) Does an ordinary Collection type (e.g. "Collection(T)" or "TCollection" where TCollection is defined by a CollectionType without the use of facets) permit null element values?

(2) Can an edm.CollectionType use the Nullable facet for scalar elements?

(3) Can an edm.CollectionType use the Nullable facet for non-scalar elements? (It seems the answer is no).