http://www.assemblee-nationale.fr/14/pdf/rap-info/i4551.pdf
https://www.gainesvilletimes.com/opinion/letter-why-mexico-border-wall-probably-waste-time-money/
http://researchbriefings.files.parliament.uk/documents/CBP-7840/CBP-7840.pdf
http://publications.iom.int/system/files/pdf/wmr_2018_en.pdf