https://taxfoundation.org/flat-tax-state-income-tax-reform/
https://www.ft.com/content/bcebd77c-057b-4fd0-bd99-b97e0e559455
http://ec.europa.eu/budget/mff/hlgor/library/reports-communication/hlgor-report_20170104.pdf
https://www.socialeurope.eu/2017/06/mercron-franco-german-tandem-back-berlins-terms/