Giving reasons for your answer, suggest an appropriate structural model for the following systems:

■ An automated ticket-issuing system used by passengers at a railway station

■ A computer-controlled video conferencing system that allows video, audio and computer data to be visible to several participants at the same time

■ A robot floor-cleaner that is intended to clean relatively clear spaces such as corridors. The cleaner must be able to sense walls and other obstructions.