gistfile1.xml

<mxml:xquery-transformer>
  <mxml:context-property key="books" value="#[flowVars['books']]" />
  <mxml:context-property key="cities" value="#[flowVars['cities']]" />
  <mxml:xquery-text>
  <![CDATA[
    xquery version "3.0";
    declare variable $document external;
    declare variable $cities external;
    declare variable $books external;
    <mixes>
    {
      for $b in fn:doc($books)/BOOKLIST/BOOKS/ITEM,
          $c in fn:doc($cities)/cities/city
            return <mix title="{$b/TITLE/text()}" city="{$c/@name}" />
    }
    </mixes>
  ]]>
  </mxml:xquery-text>
</mxml:xquery-transformer>