>> You could use
>>   as="document-node()"
>> if you want to use an "as" attribute to type annotate the variable.
>>
>> I don't think keys work with sequences, you need a root node.
>>
>
> Typing it as document-node() does not work. I am happy to leave it
> untyped as it works mind.

You will need to add xsl:document:

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

-- 
Andrew Welch
http://andrewjwelch.com