Top-forms of leading singularities in nonplanar multi-loop amplitudes Baoyi Chen, Gang Chen, Yeuk-Kwan E. Cheung, Ruofei Xie and Yuan Xin Eur. Phys. J. C, 78 2 (2018) 164 Published online: 24 February 2018 DOI: 10.1140/epjc/s10052-018-5629-z