result tree fragments and node-sets

I'm (still) a bit confused by the distinction between result tree fragments
and node-sets.

Can I do something like the following (and is it reasonable)?

<xsl:template name='foo'>
  <xsl:variable name='temp0'>
    <xsl:apply-templates mode='zero'>
  </xsl:variable>
  <xsl:variable name='temp1'>
    <xsl:apply-templates select='$temp0' mode='one'>
  </xsl:variable>
  <xsl:variable name='temp2'>
    <xsl:apply-templates select='$temp1' mode='two'>
  </xsl:variable>
  <xsl:copy-of select='$temp2' />
</xsl:template>

I had been trying to understand basic XSLT before learning about extension
functions.  Now I have read messages that seem to indicate that a RTF can't
be the argument of a select expression; only node-sets can be selected, and
the above should be written select='xx:node-set($temp0)'.  Yet by all
appearances my stylesheet is working without using the node-set extension
function.  Is this an artifact of my flat data structure?

When should I use the node-set extension function, what does it actually do,
and what more does it allow me to do?

Chris



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