if (y->avl_balance == -2)
  {
    struct avl_node *x = y->avl_link[0];
    if (x->avl_balance == -1)
      {
        w = x;
        y->avl_link[0] = x->avl_link[1];
        x->avl_link[1] = y;
        x->avl_balance = y->avl_balance = 0;
      }
    else
      {
        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;
      }
  }
else if (y->avl_balance == +2)
  {
    struct avl_node *x = y->avl_link[1];
    if (x->avl_balance == +1)
      {
        w = x;
        y->avl_link[1] = x->avl_link[0];
        x->avl_link[0] = y;
        x->avl_balance = y->avl_balance = 0;
      }
    else
      {
        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;
      }
  }
else
  return &n->avl_data;
z->avl_link[y != z->avl_link[0]] = w;

tree->avl_generation++;
return &n->avl_data;