  Gentzen's doctoral thesis marked the birth of structural proof  theory, as contrasted to the old axiomatic proof theory of Hilbert. A remarkable step ahead in the development of systems of sequent  calculus was taken by Oiva Ketonen in his doctoral thesis of 1944.  Ketonen, a student of mathematics and philosophy in Helsinki, went to Göttingen in 1938 to study proof theory with Gentzen—being the closest to a student the latter ever had. The connection  seems to have been established by Ketonen's philosophy professor Eino Kaila who had met Gentzen in 1936 in Münster. Ketonen  recollected later that Gentzen was “a sympathetic young man of  few words” who gave him an introduction to the  proof-theoretical systems and results. Ketonen's best-known discovery is a sequent calculus for classical propositional logic the logical  rules of which are all invertible, meaning that whenever a sequent is of a form that matches the conclusion of a logical rule, the  corresponding premisses, defined uniquely from the given sequent and  the rule, are also derivable. The reverse is immediate (just apply  the rule). Rules L&amp; and L⊃, for example, are modified into