Masini, A., 2-Sequent calculus: a proof theory of modalities, Annals of Pure and Applied Logic 58 (1992) 229-246. In this work we propose an extension of the Getzen sequent calculus in order to deal with modalities. We extend the notion of a sequent obtaining what we call a 2-sequent. For the obtained calculus we prove a cut elimination theorem. © 1992.
Masini, A. (1992). 2-Sequent calculus: a proof theory of modalities. Annals of Pure and Applied Logic, 58(3), 229–246. https://doi.org/10.1016/0168-0072(92)90029-Y