Challenge 2: Querying Overdue Loans with the IsOverdue() Extension Method — Possible Solution
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Program.cs:

var loans = new Repository<Loan>();
loans.Add(new Loan("001", "Alice", DateTime.Now.AddDays(7)));   // not overdue
loans.Add(new Loan("002", "Bob", DateTime.Now.AddDays(-2)));    // overdue
loans.Add(new Loan("003", "Carol", DateTime.Now.AddDays(-10))); // overdue

var overdueLoans = loans.Where(l => l.IsOverdue());

foreach (var loan in overdueLoans)
{
    Console.WriteLine(loan);
}

Output:

    Loan { Isbn = 002, Borrower = Bob, DueDate = ... }
    Loan { Isbn = 003, Borrower = Carol, DueDate = ... }

Explanation:
loans.Where(l => l.IsOverdue()) uses the extension method IsOverdue()
(csharp2-7) as the predicate passed into Repository's own LINQ-backed
Where() (csharp2-2). Only Bob's and Carol's loans, whose DueDate is
in the past, satisfy IsOverdue() and appear in the result. Each
printed loan uses the record's own free ToString() (csharp2-5), with
no manual formatting code written anywhere.

WHY THIS WORKS AS AN ANSWER
------------------------------
This combines LINQ filtering with the extension method inside the
predicate itself, exactly the composition the chapter's own capstone
demonstrates, and relies on the record's automatic ToString() for
output.