n = pa[k - 1]->bst_link[da[k - 1]] =
  tree->bst_alloc->libavl_malloc (tree->bst_alloc, sizeof *n);
if (n == NULL)
  return NULL;

n->bst_link[0] = n->bst_link[1] = NULL;
n->bst_data = item;
tree->bst_count++;
tree->bst_generation++;