Challenge 1: A Default Interface Method Inherited Automatically — Possible Solution
====================================================================

Program.cs:

interface IGreetable
{
    string Hello();
    string Bye() => "Goodbye!";   // default body
}

class Person : IGreetable
{
    public string Hello() => "Hi there!";
}

IGreetable p = new Person();
Console.WriteLine(p.Hello());
Console.WriteLine(p.Bye());

Output:

    Hi there!
    Goodbye!

Explanation:
Person only implements Hello(), the single abstract member IGreetable
actually requires. Bye() is never written in Person at all -- it's
inherited automatically from IGreetable's own default body, the same
mechanism java1-6's own Java 8 default methods use, just arriving in
C# five years later (C# 8, 2019).

WHY THIS WORKS AS AN ANSWER
------------------------------
This demonstrates a default interface method being inherited and used
without the implementing class writing any code for it, matching the
chapter's own claim that C#'s default interface methods behave the
same way as java1-6's Java 8 equivalent.