https://www.elibrary.imf.org/downloadpdf/book/9798400224119/CH004.xml
http://www.esf.be/new/wp-content/uploads/2018/07/ESF-Press-Release-on-Importance-of-Services-Trade-in-EU-UK-Future-Relastionship.pdf
http://www.ecb.europa.eu/pub/pdf/other/ebart201703_01.en.pdf
https://read.oecd-ilibrary.org/view/?ref=130_130609-v8jn83j1j3&title=Trade-facilitation-and-the-covid-19-pandemic