void st_select_lex_unit::exclude_tree()
{
  for (SELECT_LEX *sl= first_select(); sl; sl= sl->next_select())
  {
    // unlink current level from global SELECTs list
    if (sl->link_prev && (*sl->link_prev= sl->link_next))
      sl->link_next->link_prev= sl->link_prev;

    // unlink underlay levels
    for (SELECT_LEX_UNIT *u= sl->first_inner_unit(); u; u= u->next_unit())
    {
      u->exclude_level();
    }
  }
  // exclude currect unit from list of nodes
  (*prev)= next;
  if (next)
    next->prev= prev;
}