The Visser rules form a basis of admissibility for the intuitionistic propositional calculus. We show how one can characterize the existence of covers in certain models by means of formulae. Through this characterization, we provide a new proof of the admissibility of a weak form of the Visser rules. Finally, we use this observation, coupled with a description of a generalization of the disjunction property, to provide a basis of admissibility for the intermediate logics and .
"The Admissible Rules of and ." Notre Dame J. Formal Logic 59 (3) 325 - 353, 2018. https://doi.org/10.1215/00294527-3838972