document() path relative to current document

Hello I hope someone can help me with this weird question.

I am transforming multiple xmls in multiple folders with an Ant task
applying a single xsl to them.

This xsl needs to use the document() function with a URL relative 
*to the location of the currently transformed document*. The problem is that
document() works relativly from the stylesheet location.

  <xsl:variable name="path" select="concat(?PATHINFOMISSING?, @href)"/>
  <xsl:variable name="external-doc" select="document($path)"/>

Is there a way to get ?PATHINFOMISSING? / the path of the transformed
document?

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