The logic NR of Meyer's "Entailment and relevant implication" is extended to include the axiom scheme Π (A vB)-> (OA v ΠB) to create the logic R4, so named because it is a conservative extension of S4.It has been an open problem since the writing of "Entailment and relevant implication" whether Ackermann's rule y is admissible in R4.In this paper, we close this problem by proving that y is admissible in this system.
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Mares et al. (1992) studied this question.