How to detect XSLT extension usage at runtime?

I am using Xalan-J.  Since its XSLTC implementation has good performance but
doesn't support extensions, I would like to use it for sheets that don't
have extensions, and at runtime, switch to Xalan's interpretive engine for
sheets that do have extensions.  Is there an accurate programmatic way to
examine a sheet at runtime to determine whether or not it is using
extensions, so I can switch between compiled/interpreted mode?  I understand
that Xalan has a SmartTransformerFactoryImpl class that can use either
compiled/interpreted mode, but my understanding from the docs is that it
doesn't actually do any runtime checking of the sheet.

Also, does anyone know what happens if one tries to use Xalan XSLTC to
process a sheet containing extensions?  Does it report an error, or does it
just silently do the transformation and you don't get the output you
expected?

Thanks,
Kathryn Benedicto

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