extension Dictionary {
    func map<K, V>(f: (Key, Value) -> (K, V)) -> [K: V] {
        var r = Dictionary<K, V>(minimumCapacity: self.count)
        for key in self.keys {
            let (k, d) = f(key, self[key]!)
            r[k] = d
        }
        return r
    }
}