https://taxfoundation.org/flat-tax-state-income-tax-reform/
https://www.consilium.europa.eu/media/45109/210720-euco-final-conclusions-en.pdf
https://www.hesa.ac.uk/data-and-analysis/publications/finances-2015-16/introduction
https://www.cbo.gov/system/files/115th-congress-2017-2018/reports/53651-outlook.pdf