struct bst_node *s;
for (;;)
  {
    s = r->bst_link[0];
    if (s->bst_link[0] == NULL)
      break;

    r = s;
  }
r->bst_link[0] = s->bst_link[1];
s->bst_link[0] = p->bst_link[0];
s->bst_link[1] = p->bst_link[1];
q->bst_link[dir] = s;