fn main() {
  // Define a mutable variable and a reference to it
  let mut a: usize = 47;
  let aref: &usize = &a;

  // This function receives a mutable reference, and changes the original value.
  double(&mut a);
  println!("n_main = {}", a); // "n_main = 199"
}

// Accepts a mutable reference, changes the value it refers to, no return value
fn double(n_inc_ref: &mut usize) {
    *n_inc_ref *= 2;
}