https://eupinions.eu/index.php?id=12463
http://www.europarl.europa.eu/RegData/etudes/IDAN/2017/599352/EPRS_IDA%282017%29599352_EN.pdf
https://publications.parliament.uk/pa/ld201719/ldselect/ldeucom/15/15.pdf
https://assets.publishing.service.gov.uk/government/uploads/system/uploads/attachment_data/file/761852/05_December-_EU_Exit_Attorney_General_s_legal_advice_to_Cabinet_on_the_Withdrawal_Agreement_and_the_Protocol_on_Ireland-Northern_Ireland.pdf