pub fn leave_region(&self) -> Arranged<G, Tr> {
        use timely::dataflow::operators::Leave;
        Arranged {
            stream: self.stream.leave(),
            trace: self.trace.clone(),
        }
    }
Brings an arranged collection out of a nested region.

This method only applies to *regions*, which are subscopes with the same timestamp
as their containing scope. In this case, the trace type does not need to change.
differential-dataflow/src/operators/arrange/arrangement.rs