 Back to proof theory: We have to admit that we neglected some classical topics. One is the study of different proof systems and their relationships going back to Gentzen’s dissertation (1935). In their Basic Proof Theory, Troelstra and Schwichtenberg (2000) give an excellent selection, but some important calculi such as the Schütte proof systems are not covered (see, for example, Schütte 1960b, 1977). They also do not cover proof systems for temporal and modal logic, neither are substructural logics  presented.[33] 