def exactly_one(class_name, properties, conditionals):
    specified_count = mutually_exclusive(class_name, properties, conditionals)
    if (specified_count != 1):
        raise ValueError(('%s: one of the following must be specified: %s' % (class_name, ', '.join(conditionals))))
    return specified_count
def exactly_one(class_name, properties, conditionals):
    specified_count = mutually_exclusive(class_name, properties, conditionals)
    if (specified_count != 1):
        raise ValueError(('%s: one of the following must be specified: %s' % (class_name, ', '.join(conditionals))))
    return specified_count