Ah, ed. Back in the mid 80s, I had to teach a course on Unix & its tools - if I remember correctly it was called "Unix - a modern OS". One of those tools was of course ed. We couldn't use vi because the termcap/terminfo settings were screwed up for the physical serial terminals we had - I eventually fixed this & felt very pleased with myself, as those terminal config files are a b*tch.

Still, teaching bash, C and the usual suspects along with ed was very strenuous for the students, and for me - we only ran the course once.