test.rs

impl Operation for Bar {
    fn add(&self, other: int) {
    }
}

impl OtherOperation for Bar {
    fn zero(&self) -> uint {
        0
    }

}

pub trait Operation {
    fn zero(&self) -> uint;
    fn add(&self, other: int);
}

pub trait OtherOperation: Operation {
    fn one(&self) -> uint {
        self.zero() + 1
    }
}