Abstract
We discuss a ‘negative’ way of defining frame classes in (multi)modal logic, and address the question of whether these classes can be axiomatized byderivation rules, the ‘non-ξ rules’, styled after Gabbay's Irreflexivity Rule. The main result of this paper is a metatheorem on completeness, of the following kind: Ifis a derivation system having a set of axioms that are special Sahlqvist formulas and+is the extension ofwith a set of non-ξ rules, then+is strongly sound and complete with respect to the class of frames determined by the axioms and the rules.

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