mergesort
int mergesort(void *, size_t, size_t, int (*)(const void *, const void *));
mt_sort(list, mergesort, outfile);
if (mergesort(sl->leaves, sl->leaves_num,
mergesort(sl->leaves, sl->leaves_num,
#define DEFAULT_SORT_FUNC_RADIXSORT mergesort
mergesort(sl->leaves, sl->leaves_num,
mergesort(s->set, n, sizeof(*(s->set)), charcoll);
(void)mergesort(carray, n, sizeof(*carray), charcoll);