If possible, it would be very useful a way to undo someone actions.
Like in the example(file), there could be a Id.txt to give a ID for every player that entered in your server, making ID 0 the server.
The undo_mapname.txt would get the mapsize and create all the lines necessary on the file, the first command would be a letter assigned to a block type followed by the id.
Every line would start with a letter and the number 0, that is the server ID. If you undo someone changes, it would create a new undo_mapname_temp.txt and delete all the commands that the ID assigned to that player did, later check the original undo_mapname.txt to check if any of the last command of every line has changed, the ones that changed, the sever revert the block to what original was.
After that it would delete undo_mapname.txt an replace it for the undo_mapname_temp.txt