https://publications.parliament.uk/pa/cm201719/cmselect/cmsctech/705/70503.htm#_idTextAnchor001
http://www.independent.co.uk/news/uk/politics/article-127-brexit-stop-what-is-it-single-market-eu-eea-theresa-may-article-50-a7955806.html
https://committees.parliament.uk/oralevidence/2766/pdf/