Correction to: Nature Machine Intelligence https://doi.org/10.1038/s42256-018-0002-3, published online 7 January 2019.

In the version of this Article originally published, the following text was missing from the Acknowledgements: ‘Part of the research was done while S.M. was at the Institute for Advanced Study in Princeton and was supported by NSF grant CCF-1412958.’ This has now been corrected.