Shervashidze, N., Schweitzer, P., van Leeuwen, E. J., Mehlhorn, K.,
Borgwardt, K. M.: Weisfeiler-Lehman Graph Kernels,
Journal of Machine Learning Research, 12, 2359-2561 (2011)
https://www.jmlr.org/papers/volume12/shervashidze11a/shervashidze11a.pdf.