XMI12 — Association elements when no immediate references

The XMI 1.1 Specification says an Association elements is generated only if
there is no reference for the Association. But if the only references are
defined on subclasses of related types, then links could be created between
objects of the more general type, and those links cannot be passed via XMI
because there is no reference on the general type. Therefore, the
specification needs to state more precisely that an Association element is
generated in a DTD if it has no reference whose container is equal to the
type of the reference's exposedEnd.