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Published online by Cambridge University Press: 12 March 2014
In the original formulations of Lewis's S1–S5 in [1], Feys's S1°, S2° in [2], and Sobociński's S3°, S4° in [3], a primitive rule is that of replacement for strict equivalents which it is enough to express formally as:
RI: where γ′ is like γ except for having β at one place where γ has α.