assert (x->avl_balance == +1);
w = x->avl_link[1];
x->avl_link[1] = w->avl_link[0];
w->avl_link[0] = x;
y->avl_link[0] = w->avl_link[1];
w->avl_link[1] = y;
if (w->avl_balance == -1)
  x->avl_balance = 0, y->avl_balance = +1;
else if (w->avl_balance == 0)
  x->avl_balance = y->avl_balance = 0;
else /* |w->avl_balance == +1| */
  x->avl_balance = -1, y->avl_balance = 0;
w->avl_balance = 0;
