extension Sequence {
    @_inlineable
    public func prefixSC(while predicate: @escaping Predicate)
    -> UnfoldSequence<Iterator.Element, Void> {
        var iterator = makeIterator()
        return sequence(state: (), next: {
            (state: Void) -> Iterator.Element? in
//            (state: inout Void) -> Iterator.Element? in
//            _ in
            guard let e = iterator.next() else { return nil }
            return predicate(e) ? e : nil
        })
    }
}