$ time ./llil4vec big1.txt big2.txt big3.txt >out.txt

std:c++11:  2.901 secs
std:c++20:  2.850 secs

$ time OMP_NUM_THREADS=3 ./llil4vec-omp big1.txt big2.txt big3.txt >out.txt

std:c++11:  2.308 secs
std:c++20:  2.267 secs