https://ec.europa.eu/commission/sites/beta-political/files/customs_controls_0.pdf
https://institute.global/news/customs-and-exiting-european-union
https://www.eurofound.europa.eu/sites/default/files/ef_publication/field_ef_document/fomeef18008en.pdf
https://www.parliament.uk/documents/lords-committees/eu-select/Third-Interparliamentary-Forum-on-Brexit-statement.pdf