__af4list_valid
for (iter = __af4list_valid((head)->next, head); \
iter = __af4list_valid(iter->list.next, head))
for (iter = __af4list_valid((head)->next, head), \
tmp = __af4list_valid(iter->list.next, head); \
iter = tmp, tmp = __af4list_valid(iter->list.next, head))