XPath expressions in UBL / Signatures

[Stephen D Green <stephengreenubl@gmail.com>]

It would be premature to make a comment on this to the ubl-comment list
so I thought I'd bounce it off ubl-dev and see if anyone can give it a sanity
check:

We just had some debate on xml-dev about the goods and the bads about
the growing use of XPath expressions.

http://lists.xml.org/archives/xml-dev/201105/threads.html#00007

One of the upshots of this was that I put forward a view and supported it
and it didn't get completely shot down that XPath expressioons aren't yet
fully portable across different applications/implementations. The problem
is mainly with XPath expressions used without any formally defined binding
of the namespaces, i.e. if standalone and with an underspecified binding
of a) default namespaces and b) any prefixes/namespaces. The XPath
spec does not define how applications must cater for this as it expects this
to be specified in other standards which make use of XPath (such as XSLT).

Does this have ramifications for UBL and its use of XPath, particularly in
the XML signatures / signature extensions to UBL? Is it clear enough what
an application has to do about any default namespace in such an expression
and about prefix and namespace bindings? If not, I wonder if a comment
is in order. Without anything specific enough about how to execute/evaluate
an XPath expression, different applications may (validly) return different
results for the same expression and the same target/context, it seems. (I
note too that UBL uses XPath expressions in elements besides signatures
but these have been around since before 2.1.)

At the same time, it seems another upshot of the xml-dev discussion was
the news that XPath 3.0 may go some way to solving portability issues by
allowing fully qualified element names
http://lists.xml.org/archives/xml-dev/201105/msg00065.html
so maybe future UBL specs could recommend these once they become
standard and adequately supported; but that day might be some way off.
----
Stephen D Green