val_valist
va_arg(ap->val_valist, type) : (type)vec_arg(&ap->val_argv))
(void *)va_arg(ap->val_valist, uintptr_t) : \
va_copy(ap.val_valist, alist);
va_copy(ap.val_valist, alist);
va_copy(ap.val_valist, alist);
va_copy(alist, ap.val_valist);