function collapse(obj, obj2, id_img, alt_less, alt_more){
 if("none" == obj.style.display){
  obj.style.display = "block";
  obj2.style.display = "none";
  id_img.src = "imatges/arrow_up_red.gif";
  id_img.alt = alt_less;
 } else {
  obj.style.display = "none";
  obj2.style.display = "block";  
  id_img.src = "imatges/arrow_down.gif";
  id_img.alt = alt_more;
 }
 return;
}