code: def _get_report(self, with_line_nums=True):
  		
  		templ = '{} ← {}' if with_line_nums else '{}'
  
  		return '\n'.join([
  			templ.format(error.string, ','.join(map(str, sorted(set(lines)))))
  			for error, lines in self.errors.items()])
docs: Returns a report which includes each distinct error only once, together
  		with a list of the input lines where the error occurs. The latter will
  		be omitted if flag is set to False.
  
  		Helper for the get_report method.