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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 previous_session_label(self, session_label):
 """ Given a session label, returns the label of the previous session. Parameters session_label: pd.Timestamp A session whose previous session is desired. Returns ------- pd.Timestamp The previous session label (midnight UTC). Notes ----- Raises ValueError if the given session is the first session in this calendar. """

        idx = self.schedule.index.get_loc(session_label)
        if idx == 0:
            raise ValueError("There is no previous session as this is the"
                             " beginning of the exchange calendar.")

        return self.schedule.index[idx - 1]