struct Foo {
    var keys: UnsafeMutablePointer<UInt32>?
    var values: UnsafeMutablePointer<String>?
    var len: Int

    init(len: Int) {
        self.len = len
    }
}

protocol FooKeyContainer : Collection {
    associatedtype Key;
    typealias Index = Int

    var keys: UnsafeMutablePointer<Key>? { get set }
    var len: Int { get set }

    init(len: Int)
}

extension FooKeyContainer {
    public var startIndex: Int {
        return 0
    }

    public var endIndex: Int {
        return len
    }

    public func index(after pos: Int) -> Int {
        return pos + 1
    }
}


protocol FooKVContainer : FooKeyContainer, ExpressibleByDictionaryLiteral {
    associatedtype Value
    typealias Element = (key: Key, value: Value)

    var values: UnsafeMutablePointer<Value>? { get set }
}

extension FooKVContainer {
    public subscript(pos: Int) -> (key: Key, value: Value) {
        return (key: keys![pos], value: values![pos])
    }
}

extension FooKVContainer {
    public init(dictionaryLiteral newElements: (Key, Value)...) {
        self.init(len: newElements.count)
    }
}

extension Foo : FooKVContainer { }