pub fn leave_region(&self) -> Collection<G, D, R, C> {
        self.inner
            .leave()
            .as_collection()
    }
Returns the value of a Collection from a nested region to its containing scope.

This method is a specialization of `leave` to the case that of a nested region.
It removes the need for an operator that adjusts the timestamp.
differential-dataflow/src/collection.rs