code: def _binary_search_intervals(cls, intervals, index):
          
          start = 0
          end = len(intervals) - 1
          while start <= end:
              middle_index = start + ((end - start) // 2)
              middle = intervals[middle_index]
              if (middle[0] <= index) and (index < middle[1]):
                  return middle
              elif middle[0] > index:
                  end = middle_index - 1
              else:
                  start = middle_index + 1
          return None
docs: Binary search for the interval containing index,
          assuming there is such an interval.
          This function should never return ``None``.