  Kleene's work of the early 1950s also pioneered a remarkable  development in sequent calculus, namely the  “contraction-free” classical and intuitionistic calculi  today denoted by G3c and G3i. These calculi have the  property that none of Gentzen's original “structural  rules” are needed. The rule of “weakening” permits  the addition of superfluous cases and assumptions, and the rule of  “contraction” the deletion of one copy of a formula if  two were present in a list, as in