
http://eur-lex.europa.eu/legal-content/EN/TXT/PDF/?uri=CELEX:52017SC0201&from=EN

https://blogs.lse.ac.uk/europpblog/2023/10/17/why-differentiation-is-now-at-the-heart-of-europes-political-system/

https://www.wilsoncenter.org/blog-post/helmut-kohl-collection-new-resource-post-cold-war-history