> No, positions of intervals are continuously maintained as side effect
> of editing commands that change buffer text.


I still don't understand how is position maintained when user inserts
something. Say I insert some characters in interval (3), I know
total_length of (2) and (4) are updated. But how are the positions of
all the intervals after (3) -- (4), (5) and (6) updated? I don't see
relevant code in adjust_intervals_for_insertion.

      ----- (4) ---
     /       |      \
  - (2) -    |    - (6)
 /   |   \   |   /   |
(1) (2) (3) (4) (5) (6)


Sincerely, Yuan