     public func flatten() -> Set<Element.Generator.Element> {
         return self.flatMap{$0}
     }
-
+    
 }
 
 infix operator ++ { associativity left precedence 160 }

}

public protocol Flattenable {
    func flatten() -> Self    
}

extension Flattenable where Self: SequenceType {

    public func flatten() -> Self {
        return self
    }
    
}

extension Set: Flattenable {
