Business Rules
This implementation aims to simulate a library.
Each book must have an author, ISBN, title, publisher and publication date.
Each book may be loaned by a user.
Each user is identified by a full name and id.
For simplicity's sake, we'll assume that the library has an infinite number of books.
Each loan is identified by an user, a book, a laon date, a return date and an id. Which implies that only registered users are allowed to loan books. By default, the return date is 15 days, but it can be extended throguh a request, each request extends 5 days, limited to 3 request.
An user can consult the days left of the loan.
A user may return a loaned book.
The library also may receive request for registering new books.