It's clear I can't query this variable from outside the <xsl:if>
    because of the variable scope, so, what's the solution?

you don't want to define foo just if some condition holds, you want to
define foo to a value that depends on a condition, so the natural
way of encoding this is

<xsl:variable name="foo">
  <xsl:choose>
  <xsl:when test="something=false">
   ....
  </xsl:choose>
</xsl:variable>
<xsl:value-of select="$foo"/>    <!--  NOW There's A foo var!!-->


David


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