“…Moreover, nowadays these concepts are treated much more as a solution than a problem. Nevertheless, DI, as a scholarly concept, is a relatively new phenomenon in European studies, and wider: international relations; legal studies; political science; and economics (Andersen, Sitter, 2006;Tekin, 2012;Kroll, Leuffen, 2015;Kubin, 2017). It grew alongside a real-life increase in differentiation (optouts, exemptions, enhanced cooperation, constructive abstention, special clauses, additional protocols, etc.).…”