# let is_string (_:string) = ();;
val is_string : string -> unit = <fun>
# is_string 3;;
Error: This expression has type int but an expression was expected of type
         string
# is_string "3";;
- : unit = ()
# module MyUsers = Map.Make(String);;
...
# MyUsers.max_binding;;
- : 'a MyUsers.t -> MyUsers.key * 'a = <fun>
# let map = MyUsers.add "toto" 42 MyUsers.empty;;
val map : int MyUsers.t = <abstr>
# let (k, v) = MyUsers.max_binding map;;
val k : MyUsers.key = "toto"
val v : int = 42
# is_string k;;
- : unit = ()