 The combinators that are included in \(K\) (hence, in all its extensions) are \(\textsf{T}\), \(\textsf{B}\) and \(\textsf{W}\). \(\acute \varepsilon\) and \(\acute o\) are the ternary version of \(\textsf{T}\) and the quaternary version of \(\textsf{B}\), respectively. Russell’s paradox involves negation, but (either variant of) Curry’s paradox is positive, in the sense that it relies on one or two theorems of the positive implicational logic of David Hilbert. This means that if the various systems of basic logic, especially \(K^\prime\) and \(L\) are consistent, then they either cannot contain full abstraction, or the notions of implication, entailment and identity should differ from their usual counterparts. Indeed, \(K\), \(K^\prime\) and \(L\) are not extensional systems. That is, even if two expressions applied to the same expression are always equal, the equality of the applied expressions does not follow. Turning basic logic into an extensional system proved less than straightforward. Fitch’s system \(JE^\prime\) was shown to be inconsistent by Myhill, which led to a more complicated formulation of the conditions for extensional identity.