Variational principle for gravity with null and non-null boundaries: a unified boundary counter-term Krishnamohan Parattu, Sumanta Chakraborty and T. Padmanabhan Eur. Phys. J. C, 76 3 (2016) 129 Published online: 08 March 2016 DOI: 10.1140/epjc/s10052-016-3979-y