#11 Writing XMI by containment omits some objects

[James Skene]

When writing XMI by navigating the containment
hierarchy of a model, some objects are never written,
because they are not in the containment hierarchy.

This may seem obvious, but is a common problem due to a
couple of factors.

1. A bug in the XMI 1.2 spec treats attributes like
they were components, which followed literally would
lead to duplicate object specifications in the XMI.

2. There is a mismatch between XMI 1.2 and EMOF, for
which it was never intended. There is no concept of
attribute in EMOF, so there is no distinction between
attributes and non-composite references anyway.

The best solution for this is to recursively track
references as the initial hierarchy is output, then
output object hierarchies for any dangling objects at
the end.