incidentally () is the _only_ expression that is guaranteed to return
nothing and to be safe in a schema aware xslt2 processor. Other
expressions that may statically be determined empty may raise an error.
No, that's not quite true in XQuery and even less true in XSLT. XSLT
doesn't do pessimistic static type checking. Even in XQuery, you're not
allowed to raise a static error for something like
*[current-date() lt xs:date('1900-01-01')]
(which is 'guaranteed to return nothing' but cannot, strictly speaking,
'statically be determined empty')
Michael Kay
Saxonica
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