struct OddNumbersTimesFiveGenerator<S : Sequence where S.GeneratorType.Element == Int> : Generator {
    var elements : S.GeneratorType

    mutating func next() -> Int? {
        while true {
            switch elements.next() {
                case let .Some(x) where x % 2 == 0: continue
                case let .Some(x): return x * 5
                case .None: return nil
            }
        }
    }

    init(_ seq: S) {
        self.elements = seq.generate()
    }
}

let xs = [1, 5, 8, 3]
let gen = OddNumbersTimesFiveGenerator(xs)
let seq = SequenceOf<Int>({ gen })
let seq2 = map(filter(xs, { $0 % 2 != 0 }), { $0 * 5 })
println(equal(seq, seq2))