             multiplier: 1,
             constant: 0)
         superview.addConstraint(widthConstraint)
+        return widthConstraint
     }
 }
            multiplier: 1,
            constant: 0)
        addConstraint(centerHorizontally)
    }
    
    func centerChildrenHorizontally(childViews: [UIView]) {

    
    // MARK: - sizing

    func setWidthConstraint(size: CGFloat) -> NSLayoutConstraint {
        return setSizeConstraint(size, dimension: .Width)
    }
    
    func setHeightConstraint(size: CGFloat) -> NSLayoutConstraint {
        return setSizeConstraint(size, dimension: .Height)
    }
    
    private func setSizeConstraint(size: CGFloat, dimension: NSLayoutAttribute) -> NSLayoutConstraint {
        let superview = self.superview!
        setTranslatesAutoresizingMaskIntoConstraints(false)
        let widthConstraint = NSLayoutConstraint(item: self,
