Acknowledgments. I would like to thank Bob Bernecky, Nicolas Delcros, Jay Foad, and Eric Iverson for comments on the manuscript. Nick Nickolov brought to my attention comments by Dijkstra on APL [0] that I had not seen before. I contacted the author of the website and obtained a copy of Dijkstra’s letter, transcribed below: Burroughs thank [sic] you for your letter dated 31 May (?) 1981. You ...