fn complete_at(&mut self, index: usize) -> Option<B> {
        if let Some((merged, inputs)) = self.merging[index].complete() {
            if let Some((input1, input2)) = inputs {
                // Log the completion of a merge from existing parts.
                self.logger.as_ref().map(|l| l.log(
                    crate::logging::MergeEvent {
                        operator: self.operator.global_id,
                        scale: index,
                        length1: input1.len(),
                        length2: input2.len(),
                        complete: Some(merged.len()),
                    }
                ));
            }
            Some(merged)
        }
        else {
            None
        }
    }
Completes and extracts what ever is at layer `index`.
differential-dataflow/src/trace/implementations/spine_fueled.rs