I think you haven't quite gotten what "opaque token" means in this context.  The way you use tell/seek with text files is: you have read to some certain point in the file.  You call 'tell' and get back an opqaue token.  Later you can call seek with that token to get back to the place in the file that you "bookmarked".  It will never be between characters, because tell won't return such a poitner.  If you decide to call seek with something (other than 0) that you didn't get from tell, then you are on your own.