int idx = choose_merge (xsrt->runs, xsrt->run_cnt, order);
xsrt->runs[idx] = merge_once (xsrt, xsrt->runs + idx, order);
remove_range (xsrt->runs, xsrt->run_cnt, sizeof *xsrt->runs,
int idx = choose_merge (xsrt->runs, xsrt->run_cnt, order);
xsrt->runs[idx] = merge_once (xsrt, xsrt->runs + idx, order);
remove_range (xsrt->runs, xsrt->run_cnt, sizeof *xsrt->runs,