code: def branch_rate(self, filename=None):
          
          if filename is None:
              el = self.xml
          else:
              el = self._get_class_element_by_filename(filename)
  
          return float(el.attrib['branch-rate'])
docs: Return the global branch rate of the coverage report. If the
          `filename` file is given, return the branch rate of the file.