        !                Pupil                !          Librarian
~~~~~~~~!~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~!~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
        !                                     !
Borrow  !   *-----------------------------*   !
        !   | Brings the book to the desk |   !
        !   *----------------------------+*   !
        !                                +---------+
        !                                     !    v
        !                                     !   +---------------------+
        !                                     !   | Stamps the due date |
        !                                     !   ++--------------------+
        !                                     !    | within 3 weeks
        !                                     !    |
~~~~~~~~!~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~!~~~~|~~~~~~~~~~~~~~~~~~~~~~~~~
        !                       +------------------+
        !                       v             !
Return  !           +------------+            !
        !           | Returns it |            !
        !           +-----------++            !
        !                       +---------------------+
        !                                     !       v
        !                                     !      .--------------.
        !                                     !      ( Checks it in )
        !                                     !      '--------------'
