code: def sys_hist(self, name=None):
          
          if name is None:
              low = self.hist.Clone(shallow=True)
              high = self.hist.Clone(shallow=True)
              return low, high
          osys = self.GetOverallSys(name)
          hsys = self.GetHistoSys(name)
          if osys is None:
              osys_high, osys_low = 1., 1.
          else:
              osys_high, osys_low = osys.high, osys.low
          if hsys is None:
              hsys_high = self.hist.Clone(shallow=True)
              hsys_low = self.hist.Clone(shallow=True)
          else:
              hsys_high = hsys.high.Clone(shallow=True)
              hsys_low = hsys.low.Clone(shallow=True)
          return hsys_low * osys_low, hsys_high * osys_high
docs: Return the effective low and high histogram for a given systematic.
          If this sample does not contain the named systematic then return
          the nominal histogram for both low and high variations.