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