Yin, P., K. Forbus, J. Usher, B. Sageman, and B. Jee. “Sketch Worksheets: A Sketch-Based Educational Software System”. Proceedings of the AAAI Conference on Artificial Intelligence, vol. 24, no. 2, July 2010, pp. 1871-6, doi:10.1609/aaai.v24i2.18827.