Hasta La Motion!

Well nowdays my eyes are always on "The Art and Science of Digital Compositing" by Ron Brinkmann.Well in the matt creation chapter there is a piece of technique

as Ron said " Assume we are working with an image shot on a blue background. The first step the color difference method involves the creation of a new image in which this blue backing is suppressed to black. This is done by selectively substituting the green channel for the blue channel in every pixel in which the existing blue component has a greater intensity than the green component. In other words, for each pixel,

If Blue > Green
then New blue = Green
else New blue = Blue "

so Ron is telling to substitute the green channel for the blue channel and then put a condition. I tried a lot to do this in nuke. But ended scratching my head.So please all Vfx artists give it a try it will be great help.Thanks:)