GNU Zile
"GNU Zile is a text editor development kit, so that you can
(relatively) quickly develop your own ideal text editor without
reinventing the wheel for many of the common algorithms and
data-structures needed to do so.
It comes with an example implementation of a lightweight Emacs clone,
called Zemacs. Every Emacs user should feel at home with
Zemacs. Zemacs is aimed at small footprint systems and quick editing
sessions (it starts up and shuts down instantly).
More editors implemented over the Zile frameworks are forthcoming as
the data-structures and interfaces improve: Zz an emacs inspired
editor using Lua as an extension language; Zee a minimalist non-modal
editor; Zi a lightweight vi clone; and more...
Zile is a collection of algorithms and data-structures that currently
support all basic Emacs-like editing features: it is 8-bit clean
(though Unicode support is not ready yet), and the number of editing
buffers and windows is only limited by available memoryand screen
space respectively. Registers, minibuffer completion and auto fill are
available.
Zemacs implements a subset of Emacs with identical function and
variable names, continuing the spirit of the earlier Zile editor
implemented in C."