This editor was written in K&R C, please don't use ANSI C comments, function prototypes and other ANSI features, because then it won't compile anymore on decades old machines. Test modifications with clang -std=c89 to avoid such things.
This is the only vi like editor usable on CP/M-80. If you feel the urge to add features consider a fork, because if the size of the binary grows some, it also becomes useless on old 8-bit machines.