code: def highlight_line(self, payload):
          
          index_of_payload = self.target_line.lower().index(payload.lower())
          end_of_payload = index_of_payload + len(payload)
  
          self.target_line = u'{}{}{}'.format(
              self.target_line[:index_of_payload],
              self.apply_highlight(self.target_line[index_of_payload:end_of_payload]),
              self.target_line[end_of_payload:],
          )
  
          return self
docs: :type payload: str
          :param payload: string to highlight, on chosen line