Discover ambiguities or omissions in the following statement of requirements for part of a ticket-issuing system: An automated ticket-issuing system sells rail tickets. Users select their destination and input a credit card and a personal identification number. The rail ticket is issued and their credit card account charged. When the user presses the start button, a menu display of potential destinations is activated, along with a message to the user to select a destination. Once a destination has been selected, users are requested to input their credit card. Its validity is checked and the user is then requested to input a personal identifier. When the credit transaction has been validated, the ticket is issued. Ambiguities and omissions include: 1. How should the system respond when a user presses the start button? 2. What happens if a user tries to buy several tickets for the same destination together? 3. At what step are users requested to input a personal identifier? 4. Can users cancel a request if a mistake has been made? 5. How should the system respond if an invalid card is input?