I offered Ward Cunningham's TECO page in my earlier message - which has one of my favorite quotes:  "TECO Madness -- a moment of convenience, a lifetime of regret".

I also mention Cantrell's C/UNIX version: Video teco

If this was someone using teco recently, Paul's version is possible/likely as it was in C and became the teco many of used for UNIX.

For a while, I could never quite decide if I liked it more than vi, but since ed and vi were >>everywhere<< from the Cray-1 to a PC, I stopped using TECO as it sometimes took a little effort to make it work (although Paul was pretty careful) - but particularly on non-UNIX boxes (other than VMS) it might not be so easy.  Since Oscar's PiDP-10 and Angelo's PDP-1 work, I'll need an editor again, so I may have to relearn it -- be interesting to see how fast it comes back. 🤔

For systems with a C compiler, Ward Miller's s (which is a subset of vi -  https://github.com/udo-munk/s) has been my go-to. 

I was playing with getting it running on V7 since it full video for a VT-100.