AnySequence.swift

struct AnyGenerator<Element>: GeneratorType {
    init<G: GeneratorType where G.Element == Element>(_ gen: G) {
        var gen = gen
        self._next = { gen.next() }
    }
    private let _next: () -> Element?
    func next() -> Element? {
        return _next()
    }
}

struct AnySequence<Element>: SequenceType {
    init<S: SequenceType where S.Generator.Element == Element>(_ seq: S) {
        self.underestimateCount = seq.underestimateCount
        self._generate = { AnyGenerator(seq.generate()) }
    }
    private let _generate: () -> AnyGenerator<Element>
    func generate() -> AnyGenerator<Element> {
        return _generate()
    }
    let underestimateCount: () -> Int
}