@article{kleer1984,
  title={A qualitative physics based on confluences},
  author={Johan de Kleer and John Seely Brown},
  year={1984},
  journal={Artificial Intelligence},
  doi={10.1016/0004-3702(84)90037-7},
  url={https://doi.org/10.1016/0004-3702(84)90037-7}
}