__radix_tree_iter_add
iter->index = __radix_tree_iter_add(iter, 1);
iter->index = __radix_tree_iter_add(iter, offset);
iter->index = __radix_tree_iter_add(iter, 1);
iter->next_index = __radix_tree_iter_add(iter, BITS_PER_LONG);
iter->index = __radix_tree_iter_add(iter, 1);