pub fn reverse(&mut self) -> &Arrange<G, Node, Node, Diff> {
        if self.reverse.is_none() {
            self.reverse = Some(self.variable.map(|(x,y)| (y,x)).arrange_by_key());
        }
        self.reverse.as_ref().unwrap()
    }
The collection arranged in the reverse direction.
differential-dataflow/examples/graspan.rs