pub fn enter_region<'a>(&self, child: &Child<'a, G, <G as ScopeParent>::Timestamp>) -> Collection<Child<'a, G, <G as ScopeParent>::Timestamp>, D, R, C> {
        self.inner
            .enter(child)
            .as_collection()
    }
This method is a specialization of `enter` to the case where the nested scope is a region.
It removes the need for an operator that adjusts the timestamp.
differential-dataflow/src/collection.rs