NoSuchMethodException: For extension function

I'm currently using a part of your code  (related by :
http://lists.w3.org/Archives/Public/www-tag/2002Oct/0053.html)

Nonetheless I get the bellowed exception in ant:

[style] Checked both static and instance methods.
    [style] http://xml.olympe.o2t/docbook-xsl/xhtml/inline.xsl:83:54: Error!
jav
a.lang.NoSuchMethodException: For extension function, could not find method
com.
nwalsh.saxon.UnwrapLinks.unwrapLinks([ExpressionContext,] #RTREEFRAG).
    [style] Checked both static and instance methods. Cause:
java.lang.NoSuchMet
hodException: For extension function, could not find method
com.nwalsh.saxon.Unw
rapLinks.unwrapLinks([ExpressionContext,] #RTREEFRAG).

The mentioned lines 83:54 are the following

<xsl:choose>
    <xsl:when test="function-available('suwl:unwrapLinks')">
      <xsl:copy-of select="suwl:unwrapLinks($link)"/>
    </xsl:when>
    <xsl:otherwise>
      <xsl:copy-of select="$link"/>
    </xsl:otherwise>
  </xsl:choose>

Do you have an idea about this problem, I didn't find anything about that
trouble on the web,
Thanks a lot for you help
Denis