def _reversal(current, prior):
    """
    do we observe a reversal
    :param current:
    :param prior:
    :return:
         1 if red to green
         0 if none
        -1 if green to red
    """
    if current == 1 and prior == -1:
        return 1
    elif current == -1 and prior == 1:
        return -1
    else:
        return 0