function change_class(obj) {
  if (typeof obj != "undefined") {
    clname = obj.className.split("_");
	switch (clname[1]) {
	  case "i" : obj.className = clname[0] + "_a"; break;
	  case "a" : obj.className = clname[0] + "_i"; break; 
	  default : break;
	}
  }
}