This is a small utility library for highlighting code embedded in comments or strings. Examples could include doctests in Python, examples in JavaDoc, code snippets in LaTeX or reStructuredText, etc. This library doesn't implement any of that, though: it just provides the plumbing to do so.

Here's how it could look in practice:

And here's how you'd achieve that:

(font-lock-add-keywords nil
  '(("^#*\\s-*\\(>>>\\|\\.\\.\\.\\) ?\\( *\\(.+\\)$\\)"
     (0 (indirect-font-lock-highlighter 3 'python-mode)))))