REAL_IDX
#define slist_get0(list_, idx) ((list_)->list[REAL_IDX((list_), (idx))])
list->list[REAL_IDX(list, idx)] = item;
m0 = list->list[REAL_IDX(list, m)];
list->list[REAL_IDX(list, m)] = list->list[REAL_IDX(list, n)];
list->list[REAL_IDX(list, n)] = m0;
idx0 = REAL_IDX(list, idx);