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Femto Emacs is a tiny Emacs clone that can be configured with femtolisp. It is a full featured and very fast editor with syntax colloring for many languages. Like emacs, it is an extensible editor that uses femtolisp, a Scheme Lisp compatible language, for writing extensions. Femtolisp is the language used to write the Julia Language parser. There is a complete tutorial for programming in femtolisp and scheme.
Specs
Pros
Pro Very small footprint
The size of the executable file and the femtolisp library add up to only 500 k. The C source code is also small and well documented, so one can easily modify it. You can also use the source code to learn how to program a text editor.
Cons
Con Source distribution only
Femto Emacs is distributed only in source form. Therefore, you need to install ncurses, gcc and compile it with make and make install. There is no binary distribution. If you want mouse support, you need to program it in femtolisp or in C. This should not be a problem if you are a programmer, but can become an issue if you don't know Lisp or C. If you want to use femtolisp on Windows, you will need mingw and ncurses.