refrain was a misc task at 0CTF/TCTF Quals 2019 where you had to reconstruct input data from a program trace.
The program executed was `echo -n 'flag{AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA}' | convert -font Courier text:- image.png`.
Read more: https://blog.bushwhackers.ru/0ctf-2019-quals-refrain/