 Suppose that C occurs at time 0, and A and B occur at later time t. Suppose, moreover, that ACB forms a conjunctive fork. Then we can evolve the set of microstates in C forward to time \(t^\prime &gt; t\). Then \(D = U_{t^\prime}(C)\) will occur after A and B, but stand in the same probabilistic relationship to A and B that C does (see  Figure 8).  Thus ACBD will form a closed fork. Since this recipe is perfectly general, it seems that every conjunctive fork ACB with C earlier than A and B must belong to a closed fork ABCD.