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

tree->rtrb_count++;
n->rtrb_data = item;
n->rtrb_link[0] = NULL;
if (dir == 0)
  {
    if (tree->rtrb_root != NULL)
      n->rtrb_link[1] = p;
    else
      n->rtrb_link[1] = NULL;
  }
else /* |dir == 1| */
  {
    p->rtrb_rtag = RTRB_CHILD;
    n->rtrb_link[1] = p->rtrb_link[1];
  }
n->rtrb_rtag = RTRB_THREAD;
n->rtrb_color = RTRB_RED;
p->rtrb_link[dir] = n;