how to call extension function for side-effect, portab

What is the appropriate XSL instruction to call an extension for
side-effect only?  Currently I've got this in my stylesheet:

  <xsl:variable name="foo"
     select="document('file:///tmp/foo.xml')"/>
  <xsl:if test="function-available('saxon:discard-document')"
     xmlns:saxon="http://saxon.sf.net/">
    <xsl:variable name="dummy"
       select="saxon:discard-document($foo)"
       xmlns:saxon="http://saxon.sf.net/"/>
  </xsl:if>

Which not surprisingly generates a warning (from Saxon):

Warning: at xsl:variable on line 11 of file:/tmp/foo.xsl:
  A variable with no following sibling instructions has no effect

-- 
Kevin Rodgers