https://github.com/davepinch/pinchy.cc/blob/master/content/topics/works/i/isaac-asimov/the-last-question/8XOtx4sa9k4.md
The missing piece was that the experimental CPYNET protocol had no provision for appending to a file; it could just send and receive files. Adding the missing piece was a no-brainer – just a minor addition to the protocol. I don’t recall the protocol details, but appending to a file was the same as writing to a file except for the mode in which the file was opened.