xquery version "1.0";
declare namespace request="http://exist-db.org/xquery/request";
declare namespace xs="http://www.w3.org/2001/XMLSchema";
declare option exist:serialize "method=xml media-type=text/xml omit-xml-declaration=no indent=yes";

let $param1:= request:get-parameter("param1",'0')
let $param2:= request:get-parameter("param2",'0')
let $offset := 1
let $num := 3

return
<xml>
{
  for $x in subsequence(//foo/bar[
    if (($param1 = '0')
    then * (: expected ")", found "then" :)
    else (contributions/person/@val = $param1)
    and
    ($param2 = '0'))
    then *
    else (contributions/link/@val = $param2)
  ],$offset,$num)
  return $x
}
</xml>