code: def add_interval(self, precision=0):
          
          precision = precision or self.precision
          interval = round((self._stop - self._start), precision)
          self.intervals.append(interval)
          self._intervals_len += 1
          self._start = time.perf_counter()
          return self.format_time(interval)
docs: Adds an interval to :prop:intervals
              -> #str formatted time