pub fn new_from(source: &Collection<Child<'a, G, Iter>, D, Diff>) -> Self {
        use ::timely::order::Product;
        let variable = Variable::new_from(source.clone(), Product::new(Default::default(), 1));
        Relation {
            variable: variable,
            current: source.clone(),
        }
    }
Creates a new variable initialized with `source`.
doop/src/main.rs