Top-forms of leading singularities in nonplanar multi-loop amplitudesBaoyi Chen, Gang Chen, Yeuk-Kwan E. Cheung, Ruofei Xie and Yuan XinEur. Phys. J. C, 78 2 (2018) 164DOI: https://doi.org/10.1140/epjc/s10052-018-5629-z