is – according to the Glamorous Toolkit Book – "the moldable playground" and "the engine for manipulating live documents."
Introducing Lepiter: Knowledge Management + Multi-language Notebooks + Moldable Development post 
> More than a decade ago, we set out to make the inside of software systems explainable. We started from the observation that software engineering is primarily a decision making activity about lots of ever changing data points represented in code and other artifacts. We conjectured that tools are essential in this endeavor but for them to be effective they have to be adaptable to the context of each single development problem. They have to be moldable.
Lepiter is made of snippets assembled in pages. Each snippet has its own language and comes with its own editor. In its simplest form, it's a Markdown note taker with live markup that appears on demand and expandable links.
When combined with a table of contents, pages can form a book.