or these is to encapsulate all references to extension
  elements inside an <CODE><FONT face="courier, monospaced">&lt;xsl:if&gt;</FONT></CODE> element and test for the
  existance of this element before attempting to use it. XSLTC has to support
  this. Otherwise we may risk either outputting erronuous error or warning
  messages about acessing non-existing elements, or even worse, compiling in
  calls to non-existing methods in the translet, causing the JVM's verifier to
  prevent the translet from being 