pub fn from(source: &Collection<G, Edge>, step: <G::Timestamp as Timestamp>::Summary) -> Self {
        let variable = Variable::new(&mut source.scope(), step);
        EdgeVariable {
            variable: variable,
            current: source.clone(),
            forward: None,
            reverse: None,
        }
    }
Creates a new variable initialized with `source`.
differential-dataflow/examples/graspan.rs