function quantite_article(id_q, dq)
{ q = parseInt(document.getElementById(id_q).value);
  new_q = q + dq;
  OK = true;
  if(new_q == 0)
  { OK = confirm("enlever cet article de votre panier ?");
  }
  if(OK)
  { document.getElementById(id_q).value = new_q;
    document.forms['cart_quantity'].submit();
  }
}