Can you index document() nodeset with <xsl:key?

Probably been asked a hundred times but ...

I'm trying to define a key on a _massive_ nodeset that is loaded through
document() into a variable in my stylesheet. I use this nodeset to perform
all sorts of useful lookups during batch transformation of thousands of
files, but it's a bit slow with lots of :

<xsl:value-of select="$mynodeset//item[@id=$something] ...

... so I'm trying to speed it up by keying it.

The key however doesn't return anything, and I can't find any examples of
keying being done quite like this.

Is the approach at least valid? Are there any caveats like the relative
position of the document() and the <xsl:key> directives? 

Thanks



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