Keisuke Nakamura and Kiyoshi Akama and Hiroshi Mabuchi, Extending E-learning systems based on Clause-Rule model. journal = {International Journal of Computer and Information Engineering}, [online]. World Academy of Science, Engineering and Technology. April 2009, vol. 28(4). 1177 - 1183 [viewed 20 April 2024]. Available from: https://publications.waset.org/pdf/13786.