Box in Square in Cartography, Autolayout for Swift

gistfile1.swift

let percent = 0.3
layout(red, green) { red, green in
    red.center == red.superview!.center
    red.size == red.superview!.size
    green.center == red.superview!.center

    // lower priority
    green.width  == red.width  * percent ~ 100
    green.height == red.height * percent ~ 100

    green.height == green.width  // higher than ~ 100 priority
    green.width  <= red.width  * percent
    green.height <= red.height * percent
}

For some reason, the last two lines can be any priority and they work.