Comment.

src/EphemeralSequence.ml

@@ -685,6 +685,11 @@ let[@inline] lazy_reinit s =
    owns a schunk that has been reached through another sequence [s']; we only
    ask whether [s] owns a schunk that has been reached through [s]. *)
 
+(* This operation does not invalidate the existing iterators. Updating
+   [s.owner] causes the sequence to lose the ownership of its schunks
+   (including the artificial front and back schunks that an iterator
+   may create) but an iterator can tolerate this. *)
+
 let copy s =
   lazy_reinit s;
   flush_inner_chunks s;