https://www.nao.org.uk/wp-content/uploads/2018/01/PFI-and-PF2.pdf
https://taxfoundation.org/flat-tax-state-income-tax-reform/
https://curia.europa.eu/jcms/upload/docs/application/pdf/2020-05/cp200058it.pdf
https://www.boeckler.de/pdf/p_imk_report_125e_2017.pdf