Geurts, Bart, ‘Competence and complexity: the case against tractability’, 2016.unpublished · https://www.researchgate.net/profile/Bart-Geurts/publication/306914071_Competence_and_complexity_the_case_against_tractability/links/57c0095f08aed246b0f7e069/Competence-and-complexity-the-case-against-tractability.pdf