#define empty(s) (s >= sp)
#define pop(a, n, i) a = (--sp)->sa, n = sp->sn, i = sp->si
#define push(a, n, i) sp->sa = a, sp->sn = n, (sp++)->si = i
#define empty(s) (s >= sp)
#define pop(a, n, i) a = (--sp)->sa, n = sp->sn, i = sp->si
#define push(a, n, i) sp->sa = a, sp->sn = n, (sp++)->si = i