assert (x->avl_balance == -1);
w = x->avl_link[0];
x->avl_link[0] = w->avl_link[1];
w->avl_link[1] = x;
y->avl_link[1] = w->avl_link[0];
w->avl_link[0] = 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;
