Let us apply OE to Incubator to calculate what credence an observer should assign to Heads upon finding that he has a black beard. In order to do that we must first specify what reference class definition is used by the corresponding possible observer-moments (i.e. those that are in a state of knowing that they have black beards). Let’s call these possible observer-moments and (see figure 6). We need two such possible observer-moments in our model of the problem since there are two relevant possible worlds, one (w1) where Heads is true and one (w2) where Tails is true, and there is one possible observer-moment knowing it has a black beard in each of these possible worlds. For the sake of illustration, let’s assume that the reference class definition used by and is the one discussed above that places these two possible observer-moments in a separate reference from the possible observer-moments that don’t know their beard color and that places the possible observer-moment that knows it has a white beard in a third reference class on its own. (In the interest of brevity, we shall from now on frequently refer to possible observer-moments simply as “observer-moments”, when context makes it clear what is meant.)
and 
(see figure 6). We need two such possible observer-moments in our model of the problem since there are 
used by