code: def truncate(self, before=None, after=None):
          
          pxframe = self.pxs.frame
          if (before is None or before == pxframe.index[0]) and (after is None or after == pxframe.index[-1]):
              return self
          else:
              tpxs = self.pxs.frame.truncate(before, after)
              return Instrument(self.sid, InstrumentPrices(tpxs), multiplier=self.multiplier)
docs: Return an instrument with prices starting at before and ending at after