Handling of 'non-XProc' pipeline sources

Hi all,

What should happen if I pass this document to an XProc processor as the
pipeline source:

<foo/>

Do we need a static error for this ("Not an XProc pipeline"), or is the
behavior of the processor implementation-defined in this case?

Personally, I think we could have a static error for the case when the
document element of the pipeline document (or the target of p:import) is
not in the XProc namespace.

Regards,
Vojtech