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

tree->pbst_count++;
n->pbst_link[0] = n->pbst_link[1] = NULL;
n->pbst_parent = q;
n->pbst_data = item;
if (q != NULL)
  q->pbst_link[dir] = n;
else
  tree->pbst_root = n;