New Fleetwood Mac song?!

[mylittledemon]

Ok somebody help me out. Surely someone has figured this one out already...Most of us have the Very best of Fleetwood mac 2-cd set right? Ok well when you pop it on your PC, there's a little documentary you can watch that is a couple minutes long. At the very beginning Lindsey, John & Mick are jamming to some song. Lindsey is repeating this awesome guitar riff and the drums sound great. Well my question is...WHAT song is this? And why did it get cut from Say You Will?