Sequence Diagrams in XMI v1.2

I have studied the output XMI (XMI v1.2 for UML v1.4) representations
of a Seq. Diagram from a wide varity of model tools, and it seems like
the XMI standard does not represent the order in which the
messages/calls are performed. The different tools appends the sequence
as extended data (<Xmi:extension>)

Can someone confirm this? Does this mean that XMI standard does not
support Sequence Diagrams, and that SDs are nothing but Collaboration
Diagrams where the exporter appends the sequence as an Xmi:extension.

In other words: I cannot create a Sequence Diagram in XMI that is
readable in other modellers?

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