Is there any way in XSL to update a global variable?

I want to check what elements i already transformed and act accordingly. That would require me to somehow add the names of the elements to some sort of list and update it each time a new element is transformed.

But since xsl:variable isn't "variable" in the sense one would expect, i have no way of adding anything to it once it has been defined.

I have multiple included data files, so using xsl functions that only know the current set of nodes will not help.