On the different forms of the kinematical constraint in BFKL Michal Deak, Krzysztof Kutak, Wanchen Li and Anna M. Staśto Eur. Phys. J. C, 79 8 (2019) 647 Published online: 07 August 2019 DOI: 10.1140/epjc/s10052-019-7171-z