copy uuu.txt to backup_uuu.txt

read data from backup_uuu.txt
modify data
write to uuu.txt

check uuu.txt

if uuu.txt looks good: modifications done!

else:
read data from backup_uuu.txt
modify data (in some improved way)
write to uuu.txt

check uuu.txt

... (repeating until uuu.txt looks good)

delete backup_uuu.txt (or keep for historical purposes)