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Usage: exists(pathname)
This function returns true if the source file or directory
exists, false if it does not. The file or directory name can be
relative to the current source file or absolute to the source tree
root. It makes no difference if the name has a
transbuild:// scheme in front of it.
<xsl:if test="TBF:exists('background.png')">
...
</xsl:if>