move multiple option in select with jQuery

I look for this everywhere: how to move multiple option inside a select
function upfield() {
  var el = $('#selectedfield').children('[@selected]:first').prev();
  $("#selectedfield")
    .children('[@selected]')
    .each(function(){
	$(this).insertBefore(el);
      });

}

function downfield() {
  var el = $('#selectedfield').children('[@selected]:last').next();
  $("#selectedfield")
    .children('[@selected]')
    .each(function(){
	$(this).insertAfter(el);
	el = $(el).next();
      });
}