pub fn enter_region<'a>(&self, child: &Child<'a, G, G::Timestamp>)
        -> Arranged<Child<'a, G, G::Timestamp>, Tr> {
        Arranged {
            stream: self.stream.enter(child),
            trace: self.trace.clone(),
        }
    }
Brings an arranged collection into 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