https://institute.global/news/customs-and-exiting-european-union
https://www.iod.com/Portals/0/PDFs/Campaigns%20and%20Reports/Europe%20and%20trade/IoD-Customising-Brexit.pdf?ver=2018-02-15-083137-800
http://blogs.lse.ac.uk/brexit/2018/05/15/switzerland-wanted-more-immigration-controls-but-economic-self-interest-will-probably-prevail/