fn updates_for_value(&self, index: usize) -> (usize, usize) {
            let mut lower = self.vals_offs.index(index);
            let upper = self.vals_offs.index(index+1);
            // We use equal lower and upper to encode "singleton update; just before here".
            // It should only apply when there is a prior element, so `lower` should be greater than zero.
            if lower == upper {
                assert!(lower > 0);
                lower -= 1;
            }
            (lower, upper)
        }
Lower and upper bounds in `self.updates` corresponding to the value at `index`.
differential-dataflow/src/trace/implementations/ord_neu.rs