Option<string> _ = None;
Option<string> john = Some("John");
string greet(Option<string> greetee)
  			=> greetee.Match(
					None: () => "Sorry, who?",
					Some: (name) => $"Hello, {name}"
			);
greet(None)  // => "Sorry, who?"
greet(Some("John"))  // => "Hello, John"