> Instead of adding two separate angles α and β, we’ll use θ and −θ. It's not clear that the geometric proof based on pictures with positive angles α and β als

> Instead of adding two separate angles α and β, we’ll use θ and −θ. It's not clear that the geometric proof based on pictures with positive angles α and β also applies to a negative β. I think one should at least provide a separate picture of that case...