Consider a school with four classes and two periods during which the classes can be scheduled. The classes are named A, B, C, and D. Each class must be scheduled in exactly one of the two periods, and the following pairs of classes can not be scheduled at the same time: (B, C), (A, D), (C, D), (B, A).

Express the scheduling problem as a Boolean expression. That is, give a Boolean expression that is true if and only if there is a feasible schedule for the classes that satisfies all the constraints.