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Re: [xml-dev] XPath 2.0 Best Practice Issue: Graceful Degradation

    if (exists(//air...

Most of those tests could be simplified by just using the original
simple xpath and checking that the resulting double wasn't NaN.
In a real situation where one might be pulling several such values from
an input source, surely you wouldn't want to obscure the program logic
of every  expression in this way. It would be much harder to spot if the
wrong arithmetic is being used in your last exampel than in your first.

If the input is validated (against xsd or relax or schematron or any
other schema language of choice) the user will get specific error
message about missing or incorrectly typed values at an earlier stage,
before the calcualtion has been attempted.


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