This is a very interesting but not well-known editor.

Some unusual features it has:

• it is centered around outlines, this means one can divide a larger source code document into several or many smaller parts
• it supports generation of output files from such outlines - and can read them back
• it supports literate programming in a very nice way, and is a bit easier to use than org-mode
• a section of an outline can appear more than one time in the directed graph of text segments - much like a hard-linked file can appear more than once in a directory tree. For example, one can use this to keep the same text in the on-line help of a CLI program, and in a long-form help document.

Personally, I have used it extensively to generate documentation for a large commercial library around signal processing. The fact that it supports an interlinked "DAG" of text elements made it easy to cross-reference and link important stuff. For exanple, I could maintain a glossary with footnotes, and I could also export it to a braindead ancient source control system called VisualSourceSafe - let a proofreader make changes, and then import it back in, keeping the structure, and merge the corrections via git.

Overall, it can do similar stuff like Emacs + Org-Mode, but while org-modes has a huge number of features, I'd say LEO is a tad simpler.