http://library.fes.de/pdf-files/bueros/rom/16042.pdf
https://www.nao.org.uk/wp-content/uploads/2018/01/PFI-and-PF2.pdf
https://www.eca.europa.eu/Lists/News/NEWS1801_16/INSR_SSM2_EN.pdf
https://wwfeu.awsassets.panda.org/downloads/ets_revenues_report_2022___web___final.pdf