code: def total_hits(self, filename=None):
          
          if filename is not None:
              return len(self.hit_statements(filename))
  
          total = 0
          for filename in self.files():
              total += len(self.hit_statements(filename))
  
          return total
docs: Return the total number of covered statements for the file
          `filename`. If `filename` is not given, return the total
          number of covered statements for all files.