https://ec.europa.eu/commission/sites/beta-political/files/customs_controls_0.pdf
https://ir.citi.com/gps/4XZdf3%2FTki0Su8jAqZDaUQpE9uavlcQzlX1iYbkdIYylX9JIDBzNNLbLt9nzG9bLrAEB4NTeJIk%3D
http://www.cobcoe.eu/files/cobcoe-brexit-report-september-2017-e-Y4G2st.pdf
https://www.wto.org/english/res_e/booksp_e/wtr23_e/wtr23_e.pdf