layoutPriorityHelpers.swift

public extension NSLayoutConstraint.Priority {
    public static func -(lhs: NSLayoutConstraint.Priority, rhs: Float) -> NSLayoutConstraint.Priority {
        return NSLayoutConstraint.Priority(lhs.rawValue - rhs)
    }
    
    public static func +(lhs: NSLayoutConstraint.Priority, rhs: Float) -> NSLayoutConstraint.Priority {
        return NSLayoutConstraint.Priority(lhs.rawValue + rhs)
    }
}


let constraint = // Some NSLayoutConstraint object.
constraint.priority = .required - 1  // Now you can do this.