docs: def subset(self, fromdate, todate):
          '''
          Return a list of dates from the list between <fromdate> and <todate>
          (inclusive)
          '''
  #CONSIDER: raise an exception if a date is not in self
          i_from = self.index(fromdate)
          if fromdate != self[i_from]:
              # don't go back before fromdate
              i_from += 1
          i_to = self.index(todate)
          return self[i_from:i_to + 1]