This repository is the content generation and build system for the interactive textbook *Mathematics in Lean*. It is a hybrid project: primarily a set of Python scripts and a Lean 4 project that together produce the textbook in multiple formats (Sphinx RST/HTML/PDF), as well as a separate user repository with exercise stubs and solutions. The core pipeline parses Lean source files annotated with custom directives (`TEXT`, `EXAMPLES`, `SOLUTIONS`) via `scripts/mkdoc.py` to generate textbook prose and Lean exercise files. A Sphinx build then produces the final HTML/PDF documentation. The Lean source files themselves cover mathematics from basic arithmetic to measure theory, heavily leveraging the Mathlib library. The repository includes CI/CD workflows (GitHub Actions), developer environments (Devcontainers, Gitpod), and testing scripts that verify the generated Lean files compile. While not a traditional application, it functions as a specialized toolchain for creating and deploying formal mathematics educational content.