https://ec.europa.eu/commission/sites/beta-political/files/position-paper-public-procurement_en.pdf
https://publications.parliament.uk/pa/ld5801/ldselect/ldeucom/248/248.pdf
https://onlinelibrary.wiley.com/doi/epdf/10.1111/jcms.13386
https://assets.publishing.service.gov.uk/government/uploads/system/uploads/attachment_data/file/695321/An_inspection_of_exit_checks.pdf