n = tree->rtavl_alloc->libavl_malloc (tree->rtavl_alloc, sizeof *n);
if (n == NULL)
  return NULL;

tree->rtavl_count++;
n->rtavl_data = item;
n->rtavl_link[0] = NULL;
if (dir == 0)
  n->rtavl_link[1] = p;
else /* |dir == 1| */
  {
    p->rtavl_rtag = RTAVL_CHILD;
    n->rtavl_link[1] = p->rtavl_link[1];
  }
n->rtavl_rtag = RTAVL_THREAD;
n->rtavl_balance = 0;
p->rtavl_link[dir] = n;
if (y == NULL)
  {
    n->rtavl_link[1] = NULL;
    return &n->rtavl_data;
  }