p:add-xml-base - annotates all elements with their @xml:base
xsl:template - matches the fileset root element, but creates a new d:ref element instead
xsl:for-each - matches 0 or 1 d:file which has the same @href as the provided $href option
xsl:attribute - adds an @href to the d:ref with the absolute URI to the reference, resolved against the d:file/@href in case it is relative