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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 compare_lists(old=None, new=None):
    '''
    Compare before and after results from various salt functions, returning a
    dict describing the changes that were made
    '''
    ret = dict()
    for item in new:
        if item not in old:
            ret['new'] = item
    for item in old:
        if item not in new:
            ret['old'] = item
    return ret