https://www.gmfus.org/download/article/19731
http://www.congreso.es/consti/constitucion/indice/titulos/articulos.jsp?ini=155&tipo=2
http://The EU Won’t Help a Weakened Theresa May
https://ec.europa.eu/research/evaluations/pdf/archive/other_reports_studies_and_documents/hlg_2017_report.pdf