def two_queues_one_stack(queue_a_goal, queue_b_contents):
    """Given the order in which items were enqueued into queue B, and the
    desired order to have them enqueued into queue A, return a sequence of
    operations to accomplish this according to the following constraints:
        1) You cannot enqueue to queue B
        2) You cannot dequeue from queue A 
        3) You are only allowed to to use a single stack for temporary storage
    If this is possible, return the shortest list of instructions that will
    accomplish this; otherwise, return None."""

    # The three operations which can be performed.
    op = ['STACK => QUEUE_A', 'QUEUE_B => QUEUE_A', 'QUEUE_B => STACK']