https://www.eca.europa.eu/Lists/ECADocuments/Briefing_paper_MFF/Briefing_paper_MFF_EN.pdf
https://taxfoundation.org/flat-tax-state-income-tax-reform/
https://www.politico.eu/article/eu-warn-financial-risk-andorra-monaco-san-marino/
https://www.ecb.europa.eu/press/key/date/2012/html/sp120726.en.html