Item *Item_sum::set_arg(uint i, THD *thd, Item *new_val) 
{
  thd->change_item_tree(args + i, new_val);
  return new_val;
}
Item *Item_sum::set_arg(uint i, THD *thd, Item *new_val) 
{
  thd->change_item_tree(args + i, new_val);
  return new_val;
}