(* 23.1 *) 
 
(* 23.2 *) 
 
(* 23.3 *) 
 
(* 23.4 *) 
 
(* 23.5 *) 
 
(* 23.6 *) 
 
(* 
a ;; 
let (index, b) = insert a 3.2 "新しい駅" ;; 
let (index, a) = insert a 3.2 "新しい駅" ;; 
*) 
 
(* 23.7 *)