static void Main()
{
var book = new Book("Professional C# 8", "Wrox Press");
// string isbn = book.Isbn; // error: converting null literal or possible null value to non-nullable type
string?isbn = book.Isbn;
var newglory = new NewAndGlory();
string?s1 = newglory.GetANullString();
string s2 = newglory.GetAString();
// string s3 = newglory.PassAString(null); // error: cannot convert null literal to non-nullable reference or unconstrained type parameter
var old = new Legacy();
string s4 = old.GetANullString(); // no error, s1 is null!
string s5 = old.PassAString(null); // no error
var sc = new SomeClass();
string?s6 = sc.Foo();
string s7 = sc.Foo() !; // dammit
}