declare variable $local-config  external;
declare variable $global-config  external;

  let $x :=
    (for $y in fn:doc($local-config)//output-element
    return $y/@namespace,
    for $p in fn:doc($local-config)//use-prefix
    return $p,
    for $fn in fn:doc($local-config)//function-name
    return $fn/@namespace,
    if($local-config eq $global-config) then () else
       for $gy in fn:doc($global-config)//output-element
       return $gy/@namespace,
       for $gp in fn:doc($global-config)//use-prefix
       return $gp,
       for $gfn in fn:doc($global-config)//function-name
       return $gfn/@namespace
)
return
     for $z in distinct-values($x)
     return $z