(let create_user (fun (username password age) {
  (let change_username (fun (new_name)
    (set username new_name)))
  (let check_password (fun (pass)
    (= pass password)))

  (fun (&username &password &age &change_username &check_password) ()) }))

(let me (create_user "Lex" "passw0rd" 25))
(let john (create_user "John" "qwerty123" 28))

(assert (not (me.check_password "test!")) "password is not 'test!'")
(assert (!= me.username john.username) "usernames are distinct")

(assert (= me.username "Lex") "my username is Lex")
(me.change_username "Rosetta")
(assert (= me.username "Rosetta") "my username is now Rosetta!")