struct Counter {
    value: i32,
    step: i32,
}

impl Counter {
    fn bump(&mut self) {
        self.value += self.step;
    }
    fn get(&self) -> i32 {
        self.value
    }
}

fn main() {
    let mut c = Counter { value: 0, step: 3 };
    c.bump();
    c.bump();
    println!("{}", c.get());
}