https://www.un.org/esa/population/publications/migration/migration.htm
http://fedtrust.co.uk/views-from-the-federal-trust/free-movement-and-migration/
https://www.bloomberg.com/politics/articles/2017-04-27/merkel-warns-against-u-k-illusions-in-hard-line-brexit-speech?utm_content=brexit&utm_campaign=socialflow-organic&utm_source=twitter&utm_medium=social&cmpid%3D=socialflow-facebook-brexit
http://www.cityam.com/259289/tony-blairs-brexit-open-britain-speech-full