https://ec.europa.eu/commission/sites/beta-political/files/customs_controls_0.pdf
https://www.gov.uk/government/speeches/pm-commons-statement-on-eu-reform-and-referendum-22-february-2016
https://ecipe.org/blog/eu-russia-trade-since-the-war/
https://www.wto.org/english/thewto_e/whatis_e/tif_e/understanding_e.pdf