>  how do you go about making it explicit 
if you want it really explict you could use

<xsl:variable as="document-node()">
  <xsl:document>
    ... stuff


but note that the main (perhaps) difference between 

<xsl:variable name="x">
  <xsl:sequence select="foo"/>
</xsl:sequence>

and

<xsl:variable name="x" as="element()*">
  <xsl:sequence select="foo"/>
</xsl:sequence>

isn't so much the implict document node as the implict copying. in the
second case $x references foo nodes in their original trees, whereas in
the first case, the nodes are _copied_ into the temporary tree so it's
the same as if you had used copy-of rather than sequence, so you get new
nodes with new identities, and unless MK is kindly optimising stuff
away, new memory usage.

david