(: Method 2: return items from $seq1 that do not exist in $seq2 :)
(: most elegant in my opinion                                   :)
let $seq1 := (<el id="a"/>,<el id="2"/>,<el id="3"/>,<el id="4"/>,<el id="5"/>)
let $seq2 := (<el id="a"/>,<el id="b"/>,<el id="c"/>,<el id="d"/>,<el id="1"/>)

return

$seq1[ fn:not(every $id in @id satisfies $id = $seq2/@id)]