> I need to ensure that each <a2> has a preceding <a1>,

every $a2 in a2 satisfies exists($a2/preceding-sibling::a1)

or more consisely (and in XPath 1.0 syntax)

not(a2[not(preceding-sibling::a1)])

Michael Kay
http://www.saxonica.com/
 
> although there may be intervening tags.  I know that
> 
> 	name(preceding-sibling::*[1]
> 
> gives me the name of the first preceding sibling, but how do 
> I construct a test that gives me only the first preceding 
> <a1> or <a2> tag?
> 
> Thanks,
> Dave