pub fn feedback_summary<T>(level: usize, summary: T::Summary) -> PointStampSummary<T::Summary> 
where
    T: Timestamp+Default,
{
    PointStampSummary {
        retain: None,
        actions: std::iter::repeat(Default::default()).take(level-1).chain(std::iter::once(summary)).collect(),
    }
}
Produces the summary for a feedback operator at `level`, applying `summary` to that coordinate.
differential-dataflow/src/dynamic/mod.rs