* SORT_INPUT_PRESORT custom presorter. Calls function
* int PREFIX_presort(struct fastbuf *dest, void *buf, size_t bufsize);
* to get successive batches of pre-sorted data.
* SORT_INPUT_PRESORT custom presorter. Calls function
* int PREFIX_presort(struct fastbuf *dest, void *buf, size_t bufsize);
* to get successive batches of pre-sorted data.