docs: def started_after(self, date):
          '''
          Leaves only those objects whose first version started after the
          specified date.
  
          :param date: date string to use in calculation
          '''
          dt = Timestamp(date)
          starts = self.groupby(self._oid).apply(lambda df: df._start.min())
          oids = set(starts[starts > dt].index.tolist())
          return self[self._oid.apply(lambda v: v in oids)]