Sure, here’s Dijkstra on it[1]. The X1[2] was a significantly more limited machine than the PDP-11. I believe that work predates Unix by nearly a decade.

[1] https://www.cs.utexas.edu/users/EWD/transcriptions/EWD13xx/E...

[2] https://ub.fnwi.uva.nl/computermuseum//X1.html