OpenMp version:
$ time ./llil4vec_p big1.txt big2.txt big3.txt >vec.tmp
llil2vec (fixed string length=6) start
get_properties      time : 0.853868 secs
emplace set sort    time : 0.972116 secs
write stdout        time : 0.875946 secs
total               time : 2.70229 secs

real    0m3.041s
user    0m5.553s
sys     0m0.745s