Consider the following UML Class diagram of Car Rental System, briefly explain the class diagram in your own words. Also, write the following OCL expression:

1. Write an invariant to restrict BlackListed people from renting cars. Also, write an OCL expression to initializes premium attribute of Customers class with false value.
2. For Customer class write OCL expression which describes that premium members get a 30% discount while nonpremium members get 15% if they have at least rented high category cars six times while the rest of the customers get no discount at all.
3. Write OCL expression for query operation that returns true if the car on which the operation is executed is the most popular in the rental system.
4. Write OCL expression for an operation which describes the business logic behind the creation of a new rental (customer) in the car rental system.