Song, Y., J. Wang, T. Lukasiewicz, Z. Xu, and M. Xu. “Diversity-Driven Extensible Hierarchical Reinforcement Learning”. Proceedings of the AAAI Conference on Artificial Intelligence, vol. 33, no. 01, July 2019, pp. 4992-9, doi:10.1609/aaai.v33i01.33014992.