http://library.fes.de/pdf-files/id/ipa/13354.pdf
http://www.euronews.com/2017/09/19/portugal-in-cash-for-passports-scandal?utm_source=newsletter&utm_medium=en&utm_content=portugal-in-cash-for-passports-scandal&_ope=eyJndWlkIjoiZjU3Y2JhYmUwMGUxZDFiNTUxOTU1ZDljNzExNTI1MmEifQ==
http://www.consilium.europa.eu/en/press/press-releases/2017/04/29-euco-brexit-guidelines/
https://assets.publishing.service.gov.uk/government/uploads/system/uploads/attachment_data/file/695321/An_inspection_of_exit_checks.pdf