Comment: version and forward compatibility

(I only found out about XProc yesterday through an announcement to the XSL 
list.  Apologies for coming late to the party; it looks like a good one.)

I could find no mention in the 20 September 2007 draft of how an XProc 
pipeline instance indicates the version of XProc that it was written for 
(something akin to xsl:stylesheet/@version for XSLT).

I can see this mattering when XProc 1.1 or 2.0 or whatever comes along, 
and people try to run XProc 2.0 pipelines with an XProc 1.0 processor. 
What is the expected behaviour when this happens?

One of the wisest things, IMHO, that the XSLT working group did was to 
nail down forward-compatible behaviour for XSLT 1.0 processors running 
future-versioned stylesheets.  I can see this being equally important for 
XProc.

Is this a legitimate concern?  If not, why not?

-- 
Deborah Pickett
Information Architect, Moldflow Corporation, Melbourne
Deborah_Pickett@moldflow.com