code: def sys_hist(self, name=None, where=None):
          
          total_low, total_high = None, None
          for sample in self.samples:
              if where is not None and not where(sample):
                  continue
              low, high = sample.sys_hist(name)
              if total_low is None:
                  total_low = low.Clone(shallow=True)
              else:
                  total_low += low
              if total_high is None:
                  total_high = high.Clone(shallow=True)
              else:
                  total_high += high
          return total_low, total_high
docs: Return the effective total low and high histogram for a given
          systematic over samples in this channel.
          If a sample does not contain the named systematic then its nominal
          histogram is used for both low and high variations.
  
          Parameters
          ----------
  
          name : string, optional (default=None)
              The systematic name otherwise nominal if None
  
          where : callable, optional (default=None)
              A callable taking one argument: the sample, and returns True if
              this sample should be included in the total.
  
          Returns
          -------
  
          total_low, total_high : histograms
              The total low and high histograms for this systematic