getIndexOf(sum) {
        this._ensureValid();
        const idx = this._indexBySum[sum];
        const viewLinesAbove = idx > 0 ? this._prefixSum[idx - 1] : 0;
        return new PrefixSumIndexOfResult(idx, sum - viewLinesAbove);
    }
@returns `result`, such that `getPrefixSum(result.index) + result.remainder = sum`