Pairwise sequence method for Swift

pairwise.swift

extension Sequence {
    /// `[e1, e2, …, eN]` → `[(nil, e1), (e1, e2), …, (eN-1, eN), (eN, nil)]`
    func pairwise() -> UnfoldSequence<(Self.Element?, Self.Element?), ((Self.Element?, Self.Element?)?, Bool)> {
        var iter = self.makeIterator()

        var next1: Element? = nil
        var next2 = iter.next()

        return Swift.sequence(first: (next1, next2)) { prevpair -> (Element?, Element?)? in
            guard let nv2 = next2 else { return nil }
            next1 = nv2
            next2 = iter.next()
            return (next1, next2)
        }
    }
}