Also you're solution works pretty good i tried to declare the param on
  top-level, but doing this i'm not able to use <xsl:with-param>. I
  wonder if it is possible in xslt to declare "global" variables/params
  that can be access everywhere in the stylesheet?


yes, any xsl:param or xsl:variable that is declared as a child of
xsl:stylesheet is a global variable that may be accessed from any
template without it being declared as a parameter to the template.
(If a local variable of the same name is declared on a template it will
make the global variable inaccessible)

David



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