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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def retrieve_exposure_classes_lists(exposure_keywords):
 """Retrieve exposures classes. Only if the exposure has some classifications. :param exposure_keywords: exposure keywords :type exposure_keywords: dict :return: lists of classes used in the classifications. :rtype: list(dict) """

    # return None in case exposure doesn't have classifications
    classification = exposure_keywords.get('classification')
    if not classification:
        return None
    # retrieve classes definitions
    exposure_classifications = definition(classification)
    return exposure_classifications['classes']