function prd_add_qte(champ, action, step) {
  step = Math.abs(parseInt(step));
  qtei = $("#"+champ).val();
  qtei = parseInt(qtei);
  qtei = (Math.ceil(qtei / step)) * step;
  if (action == '+') {
    qte = qtei + step;
  } else 
  if (action == '-') {
    qte = qtei - step;
  } else 
    qte = qtei;
  if (qte > 999) { qte = qtei; }
  if (qte < 0) { qte = 0; }
  $("#"+champ).val(qte);
  return true ;
}