But... why? If you wanted to express their equation in terms of tau (that is, 2pi), you could just set the first term of the right hand side to 10/phi instead o

But... why? If you wanted to express their equation in terms of tau (that is, 2pi), you could just set the first term of the right hand side to 10/phi instead of 5/phi. In fact, throughout their derivation there are points where a tau-based version would be a bit cleaner (though of course there are other points where tau might be a bit messier).