(: Method 1:  return items from $seq1 that do not exist in $seq2 :)
(: lots of people do this one as its conceptually clear          :)

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

for $item in $seq1
return
  if($seq2[@id eq $item/@id]) then
    ()
  else
    $item