struct ZipSequence<S1: Sequence, S2: Sequence, Ss: variadic Sequence> { private let s1: S1 private let s2: S2 private let ss: Ss // used as a type here, in a property declaration... // ... and here in a function signature init(_ s1: S1, _ s2: S2, _ ss: Ss) { (self.s1, self.s2, self.ss) = (s1, s2, ss) } }