[remark](https://github.com/remarkjs/remark) is a pluggable markdown parser and compiler. It's part of the [unified](https://unifiedjs.com) so it offers a wide array of libraries to make writing plugins with the markdown AST ([MDAST](https://github.com/syntax-tree/mdast)) more developer friendly. We'll use `unist-util-visit` to traverse heading nodes in a Markdown file and prefix all `h1`s with the text "BREAKING" using a TDD workflow.