pub fn set(self, result: &Collection<G, D, R, C>) -> Collection<G, D, R, C> {
        let step = self.step;
        result
            .inner
            .results_in(step)
            .connect_loop(self.feedback);

        self.collection
    }
Adds a new source of data to `self`.
differential-dataflow/src/operators/iterate.rs