Roy Dyckhoff Luis Pinto Permutability of Proofs in Intuitionistic Sequent Calculi. 141-155 1999 212 Theor. Comput. Sci. 1-2 db/journals/tcs/tcs212.html#DyckhoffP99 http://dx.doi.org/10.1016/S0304-3975(98)00138-8