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Published online by Cambridge University Press: 12 March 2014
Kanger [4] gives a set of twelve axioms for the classical prepositional calculus which, together with modus ponens and substitution, have the following nice properties:
(0.1) Each axiom contains =⊃, and no axiom contains more than two different connectives.
(0.2) Deletions of certain of the axioms yield the intuitionistic, minimal, and classical refutability1 subsystems of propositional calculus.