This is an emulation of the old [teco](https://en.wikipedia.org/wiki/TECO_%28text_editor%29) editor.
It was written by Matt Fichtenbaum and posted in 1986.
The C has been modernised, VAX support has been dropped, and
an undo facility has been added.
I still use it when needing to deal with text where changes
need to span over line breaks.