Zhang, X., J. V. den Bussche, K. Wang, H. Zhang, X. Yang, and Z. Feng. “On the Expressivity of ASK Queries in SPARQL”. Proceedings of the AAAI Conference on Artificial Intelligence, vol. 34, no. 03, Apr. 2020, pp. 3057-64, doi:10.1609/aaai.v34i03.5700.