     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 {

// MARK: Attributes

public enum Horizontal {
    case centerX, left, right, leading, trailing
}

extension Horizontal: SingleOffset, SingleAttribute {
