a.swift

struct ReductionsSequence<Base: Sequence, T>: Sequence, IteratorProtocol {
    typealias Element = T

    private var iterator: AnyIterator<Base.Element>
    private var current: T
    private let reducer: (T, Base.Element) -> T

    init(sequence: Base, initialValue: T, reducer: @escaping (T, Base.Element) -> T) {
        iterator = AnyIterator(sequence.makeIterator())
        self.reducer = reducer
        current = initialValue
    }

    mutating func next() -> ReductionsSequence<Base, T>.Element? {
        guard let next = iterator.next() else {
            return nil
        }

        let previous = current
        current = reducer(current, next)
        return previous
    }
}

extension Array {
    func reductions<T>(initialValue: T, _ reduce: @escaping (T, Element) -> T) -> AnySequence<T> {
        return AnySequence(ReductionsSequence(sequence: self, initialValue: T, reducer: reduce))
        // Cannot convert value of type '(T, Array<Element>.Element) -> T' (aka '(T, Element) -> T') to expected argument type '(_, _) -> _'
    }
}