/// <inheritdoc cref="State{T}.Sum"/>
        public override FrequenciesAndNumRows Sum(FrequenciesAndNumRows other)
        {
            IEnumerable <string> columns = Frequencies.Schema().Fields
                                           .Select(field => field.Name)
                                           .Where(field => field != AnalyzersExt.COUNT_COL);

            IEnumerable <Column> projectionAfterMerge = columns
                                                        .Select(col =>
                                                                Coalesce(Col($"this.{col}"), Col($"other.{col}")).As(col))
                                                        .Append(
                (AnalyzersExt.ZeroIfNull($"this.{AnalyzersExt.COUNT_COL}") +
                 AnalyzersExt.ZeroIfNull($"other.{AnalyzersExt.COUNT_COL}")).As(AnalyzersExt.COUNT_COL));


            Column joinCondition = columns.Aggregate(NullSafeEq(columns.First()),
                                                     (previous, result) => previous.And(NullSafeEq(result)));


            DataFrame frequenciesSum = Frequencies
                                       .Alias("this")
                                       .Join(other.Frequencies.Alias("other"), joinCondition, "outer")
                                       .Select(projectionAfterMerge.ToArray());


            return(new FrequenciesAndNumRows(frequenciesSum, NumRows + other.NumRows));
        }