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

tree->prb_count++;
n->prb_link[0] = n->prb_link[1] = NULL;
n->prb_parent = q;
n->prb_data = item;
if (q != NULL)
  q->prb_link[dir] = n;
else
  tree->prb_root = n;
n->prb_color = PRB_RED;

