pub fn flat_map_ref<I, L>(&self, logic: L) -> Collection<G, I::Item, Tr::Diff>
        where
            I: IntoIterator,
            I::Item: Data,
            L: FnMut(Tr::Key<'_>, Tr::Val<'_>) -> I+'static,
    {
        Self::flat_map_batches(&self.stream, logic)
    }
Extracts elements from an arrangement as a collection.

The supplied logic may produce an iterator over output values, allowing either
filtering or flat mapping as part of the extraction.
differential-dataflow/src/operators/arrange/arrangement.rs