     var nsAttributes: (NSLayoutAttribute, NSLayoutAttribute) {
         switch self {
         case .horizontally: return (.left, .right)
+        case .leadingToTrailing: return (.leading, .trailing)
         case .vertically: return (.top, .bottom)
         }
     }

// MARK: Stack

public enum Stack {
    case vertically, horizontally
}

extension Stack: DoubleAttribute {

        case .centerX: return .centerX
        case .left: return .left
        case .right: return .right
        case .leading: return .leading
        case .trailing: return .trailing
        }
    }
}
