(: Method 3: 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"/>)
let $s2ids := distinct-values($seq2/@id)
return
  $seq1[not(@id=$s2ids)]