Calling extension fuction error

I call evaluate function of XALAN but il always issue an  errors at
runtime

Call to extension function failed: http://xml.apache.org/xalan

I have set name space to
xmlns:xalan ="http://xml.apache.org/xalan"
exclude-result-prefixes="xalan"
and  bsf at classpath.

I can't find out the reason, any one can help?

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