Well, it is not that difficult: Search in module system for fmReadWrite and you
will see
fmReadWrite, ## Open the file for read and write access.
## If the file does not exist, it will be
## created. Existing files will be cleared!
RunSo file is erased at startup! I guess this behaviour is not Nim specific, I may have seen it in other languages, it may be related to OS. Well, may be not that easy to detect for people without some experience and a will to discover unexpected result on their own.
