//@ `println!` is again a macro, where the first argument is a *format string*. For
//@ now, you just need to know that `{}` is the placeholder for a value, and that Rust
//@ will check at compile-time that you supplied the right number of arguments.
fn print_number_or_nothing(n: NumberOrNothing) {
//@ `println!` is again a macro, where the first argument is a *format string*. For
//@ now, you just need to know that `{}` is the placeholder for a value, and that Rust
//@ will check at compile-time that you supplied the right number of arguments.
fn print_number_or_nothing(n: NumberOrNothing) {