https://docs.iza.org/dp16728.pdf
https://blogs.lse.ac.uk/brexit/2018/10/30/after-brexit-the-uk-should-be-able-to-re-enter-the-eu-if-it-wishes-to-do-so/
https://www.socialeurope.eu/wp-content/uploads/2018/05/BP1-Devolution.pdf
https://www.migrationpolicy.org/article/spain-immigration-system-evolution?eType=EmailBlastContent&eId=a1fcdaaa-93ef-4a35-9397-e325534c6592&mc_cid=e4eef57c9d&mc_eid=d10a99d01e