“You are not expected to understand this.” Those seven words, which appeared in the source code of Unix’s sixth edition in 1975, have since been reproduced by computer geeks on T-shirts, mugs, jumpers and, with the inevitability of an unloved season, tote bags. The phrase’s cultural afterlife, and the reasons it has become a rallying cry for programmers everywhere, inspire the titular essay in a new collection on the most important lines of computer code in human history, and how