http://www.diw.de/documents/publikationen/73/diw_01.c.562050.de/17-29.pdf
https://blogs.lse.ac.uk/brexit/2019/12/03/a-missed-opportunity-revisiting-the-euro-referendum-that-never-was-from-a-historical-perspective/
https://theconversation.com/eu-enlargement-what-does-the-future-hold-227716