OK, all you EMACS devotees. Who among you know what EMACS stands for? “Editor MACroS”, that’s right. But what language were these macros originally written in? Thanks to Good Math, Bad Math, you can now read all about the World’s Greatest Pathological Language: TECO. I first encountered it in 1970, when the Essex University PDP-10 was delivered.
This extract will pique your curiosity, or send you running for cover. I’ve modified it a bit from the original piece, because when I tried to quote it, some of the special characters were interpreted as bits of HTML. Besides, it looked all wrong to me; anyone who actually used TECO will remember that the ESCAPE key was echoed as $:

[…] The print command to print a string is control-A; so the TECO hello world program is: “^AHello world^A$$“. Is that pathological enough?
Commands to remove text include things like “D” to delete the character after the pointer; “FD”, which takes a string argument, finds the next instance of that argument, and deletes it; “K” to delete the rest of the line after the pointer, and “HK” to delete the entire buffer.
To insert text, you can either use “I” with a string argument, or TAB with a string argument. If you use the tab version, then the tab character is part of the text to insert.

I still remember the illicit thrill that ran through the Computing Centre when we learned that someone had created a TECO macro to invert a matrix. In retrospect, I blame the freely-available hallucinogens and too much Hunt the wumpus….