https://blogs.lse.ac.uk/brexit/2019/10/09/northern-ireland-depends-on-the-eus-fluid-borders-for-survival/
https://www.spectator.co.uk/article/full-text-top-uk-brexit-negotiator-david-frost-on-his-plans-for-an-eu-trade-deal
http://www.rug.nl/research/portal/files/41711829/cesifo1_wp6448.pdf