XMI13 — XMI Schema FTF issue - referenceless composition does not export components

This I think brings the spec into line with what implementations do in
practice!

It's possible to create a composition without having a Reference from the
composite to the component. This is not obscure - a prime example of this is
in UML: a ModelElement owns Tags but has no reference to them.
The XMI production rules talk only about traversing composite references.

Because the components we're talking about DO participate as a component in
a composition link then these components will neither be starting points for
generating XML nor exported as part of their composition (since they are not
referred to by a reference).

Proposed Resolution
-------------------

1. Change the description of production rule 2a in 6.3.2 (p6-101) from:
"The content elements are the XML representations of top level objects,
classifier level attributes, and other links."
To:
"The contents are the XML representations of top level objects, classifier
level attributes, and other links. The top level objects will include those
which have a composite link with no reference from the composite to
component."

2. Add a new sentence after the first to rule 5 in 6.3.2 (p6-107) changing
the start of the rule from:
"The contents of an object are the attributes, references, and compositions
that are serialized as XML elements, as well as the extensions."
To:
"The contents of an object are the attributes, references, and compositions
that are serialized as XML elements, as well as the extensions. Note that
'contents' (component objects which are reached via composite links) without
a composite reference are not subject to this production rule and so not
written as nested elements: instead they are written as top-level elements."

see below