XMI12 — XMI 1.2 Issue - Association elements with references

There are several problems related to the specification of Association
elements (representing Links).

1) At the moment Association elements are only generated in DTDs/Schemas for
referenceless associations.
This precludes the use of Association instances in XMI files to link
existing/external objects: a technique I have certainly found useful even
when those objects do have references. There seems to be little reason for
not generating the Association elements.

2) The description of DTD design principles in 3.6.6 refers to association
roles. Presuming this is intended to mean association ends, the text is not
accurate since the element name generated is based on the reference name for
the class which may differ from the association end name.

3) Section 3.6 does not mention the creation of Association elements even
for the existing referenceless Association case.

4) The DTD and document generation rules for Association elements are
unnecessarily heavyweight and inconsistent with the treatment of references
on classes. For example it does not make sense for an AssociationEnd element
to have the fixed xmi attributes; and it should be possible to dispense with
them completely: letting an Association element refer to the linked elements
using XML attributes. e.g. for association A with ends b and c, a link could
be represented as follows (b1 and c1 are xmi.id's): <A b=b1 c=c1 />).