[DR702] Evolution

I would prefer to see this phrased in terms of the XP application
determining whether a given message conforms to a version of XP that the
application recognises rather than in terms of comparison of twos messages.
I don't think evolution is defined in terms of messages being compatible
with each other but in terms of messages conforming to a given version of
XP.

I'm also having some difficulty understanding what the last sentence means.
How can an XP envelope *mandate* that an application be extended?

Gudge