$($a$)$ Explain what is meant by Design by Contract $($DbC$).$ Elaborate on how a contract is

affected by subclassing$/$polymorphism$.$

$(9$ marks$)$

$($b$)$ Within the context of $DbC,$ comment on benefits and obligations for both client code

and provider code. Mention when exceptions might be appropriate.

$(6$ marks$)$

$($c$)$ Given the class diagram below, write an Object Constraint Language $($OCL$)$ contract

invariant for ReceivablesAccount which states that no invoice can be in both

processedInvoices and unprocessedInvoices collections at the same time.

Write an OCL contract that you deem appropriate to express the business logic

ProcessInvoices $()$ operation of the class ARProcessor.