4. (a) Software is required to keep a record of the passengers who have boarded an aircraft. An aircraft has a maximum capacity. Provide an UML class diagram which specifies the state of the software system and which also shows two operations which modify the state. Describe briefly a class invariant which applies here but which a class diagram cannot show. Also using natural language or set notation, specify one of the operations more precisely by providing an appropriate contract for it. (7 marks) 7 ) 6) Give a UML-based Specification Environment (USE) specification for the class diagram from part (a) along with an OCL contract for one operation. Do not consider the implementation of the operation at this stage, just describe what is done rather than how. (10 marks) (c) Using SOIL provide an abstract implementation of the operation described in part (b). create some test objects which will allow you to test your specification in USE. Explain how you could then test your model using the USE command line interface. (8 marks)