[xsl] grouping in XPATH with parenthesis

[Dan Diebolt <dandiebolt@xxxxxxxxx>]

I recently saw an XPATH expression (in Dave Obasanjo's "Things to
Know and Avoid When Querying XML Documents") that included parentheses 
used for grouping:

(//*)[position()=1]/@on-loan

I had to scratch my head wondering if I have ever seen parentheses
used in this fashion. Is anyone familiar enough with this usage
to explain when parenthesis would be needed and where in the
specs they are even allowed?

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