variable with document() reference

I really should know the answer to this, but I can't seem to get a handle on it.

If I reference an external document in a variable, like...

<xsl:variable name="xln-source" select="document($file)" />

...and then need to loop through it, like...

<xsl:for-each select="$xln-source//set[descendant::start/@xlink:href = $match]">
   <!-- do something -->
</xsl:for-each>

...is the external document being loaded in the variable or it the
for-each call?  I have a different for-each later requiring the same
document and I would like to load it only once.

I'm actually thinking that I may need to move this to keys anyways,
but it's bothering me that I can't answer this question.

Thanks,
-m