https://www.publications.parliament.uk/pa/ld201617/ldselect/ldeucom/78/78.pdf
https://hansard.parliament.uk/commons/2018-03-07/debates/65828E0C-3CC7-43AE-B5D2-B5F1B5D42A4C/EuropeanUnionCitizenship
https://blogs.lse.ac.uk/brexit/2019/04/15/brexit-lessons-from-the-silesian-backstop-of-1919-25/