if (subheight[1] - subheight[0] != node->avl_balance)
  {
    printf (" Balance factor of node %d is %d, but should be %d.\n",
            d, node->avl_balance, subheight[1] - subheight[0]);
    *okay = 0;
  }
else if (node->avl_balance < -1 || node->avl_balance > +1)
  {
    printf (" Balance factor of node %d is %d.\n", d, node->avl_balance);
    *okay = 0;
  }