docs: def find_index(segmentation, stroke_id):
      """
      >>> find_index([[0, 1, 2], [3, 4], [5, 6, 7]], 0)
      0
      >>> find_index([[0, 1, 2], [3, 4], [5, 6, 7]], 1)
      0
      >>> find_index([[0, 1, 2], [3, 4], [5, 6, 7]], 5)
      2
      >>> find_index([[0, 1, 2], [3, 4], [5, 6, 7]], 6)
      2
      """
      for i, symbol in enumerate(segmentation):
          for sid in symbol:
              if sid == stroke_id:
                  return i
      return -1