[relax-ng-comment] Validating PI's and DOCTYPE

[Peter Wilson <PWILSON@GORGE.NET>]

A document with invalid PI's is just as broken as one with an invalid
element or attribute.
It seems a shame to have to invent a completely parallel system to
RelaxNG to validate the PI's in a document.

PI's commonly can have a very similar structure to elements when the PI
data takes the form of name="value" pairs.
Is there some way to extend RelaxNG so that the validity of PI's can be
specified?

This also applies to the system and public identifiers on a doctype (I
realize that the internalSubset is way out of bounds here).