docs: def persistent_oid_counts(self, dates):
          '''
          Counts how many objects (identified by their oids) existed before
          or on a given date.
  
          :param dates: list of the dates the count should be computed.
          '''
          total = pd.Series([self.on_date(d)._oid for d in dates],
                            index=dates)
          for i in range(1, total.size):
              a1 = total[total.index[i - 1]]
              a2 = total[total.index[i]]
              total[total.index[i]] = list(set(a1) | set(a2))
          return total.apply(len)