<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def aggregations(self):
 """ Override parent method. Obtain list of the terms and their corresponding values using "terms" aggregations for the previous time period. :returns: a data frame containing terms and their corresponding values """


        prev_month_start = get_prev_month(self.end, self.query.interval_)
        self.query.since(prev_month_start)
        self.query.get_terms("author_name")
        return self.query.get_list(dataframe=True)