Keeps the team's memory: where what you learned goes (your memory, shared with the team when it is about the project, a lesson in .opencastle/lessons/, or a team skill for a procedure), and how lessons are cited, re-verified and graduated. Use when you confirm something true for anyone working in the repository (a fix after a failure, a convention, a decision, a quirk, a correction about the code), when you work out a procedure no skill covers, when you touch a lesson copied from memory, when doctor reports a lesson's cited code changed, or when the lessons index grows long.