If we have a path where b is evaluated as 1 0 0 0 ... then F b is 1 0 0 0 ... also, since at the initial point of time, we consider the path 1 0 0 0 ... and find a point of time where b holds. At the next point of time, and all others that follows, we consider the path 0 0 0 ... and there is no point of time where b holds. Thus F b is false there.