Flander's bus company "De Lijn" has a number of busses which are used for different bus lines with different destinations. The company possesses a number of trajectories which have to be approved by town authorities.  When the company asks for approval, that approval may be refused without appeal or town authorities may ask the bus company to change the trajectory upon which the company may ask again for approval.  This procedure may be repeated a number of times.  When approval is given, the trajectory is included in the business of the company to become operational at the beginning of the next calendar year.  From that time on, busses can be assigned to the trajectory.  The trajectory may be suspended for some time for all kinds of reasons.  At a certain moment in time, the trajectory is cancelled.  
Buses get a 'line' number. A 'Line' is mostly the same as a trajectory, but not entirely. For the sake of this case, we assume that a line groups a number of overlapping trajectories. E.g., in the late evening, the last section of a line may be skipped (trajectory is shorter), or there is a bifurcation with two different end points at one end, which are taken alternatively. E.g. in Leuven, one has "Line 2 Campus" and "Line 2 Boskant" which follow partly the same and partly different routes.
Bus company "De Lijn" wishes to test a new type of season ticket, called MENSO.  For the purpose of the MENSO project, all bus lines have been assigned a category.  A MENSO ticket is valid during one year and the price of a trip depends of the category of line you are travelling with.  As a result, the price of the ticket is not known in advance and MENSO users will be invoiced at the end of each month for the trips of the past month.  
In order to pay with a trip with MENSO, the user simply introduces his/her card in the ticket machine next to the bus door upon entering the bus.  The system will then register time, date and line of the trip.  If the user has forgotten his/her menso-card, (s)he has to pay in the usual way: by buying a ticket of using another type of card.
The time, date and line information is collected in order to generate the monthly invoices. Depending on the total invoice amount, the customer will be granted a discount of up to 20%.  Invoicing and payment happens in any order: a customer might receive the next invoice before the first one is paid.
Through the MENSO system, "De Lijn" hopes to gather interesting information on the behaviour of his customers.  When subscribing to MENSO, the customer will be requested some personal data such as home address, office address and working hours.  By combining this information with the information generated by using the MENSO card, De Lijn hopes to detect patterns in travelling habits, allowing to optimise the routes and frequency of the bus lines.