code: public void addValueCombinationsCrosstabDimension(final Map<ValueCombination<Number>, Number> valueCombMapList,
              final Crosstab<Number> partialCrosstab) {
  
          final CrosstabNavigator<Number> nav = new CrosstabNavigator<>(partialCrosstab);
          final CrosstabDimension columnDimension = partialCrosstab.getDimension(BooleanAnalyzer.DIMENSION_COLUMN);
          final CrosstabDimension measureDimension = partialCrosstab.getDimension(BooleanAnalyzer.DIMENSION_MEASURE);
  
          final List<String> columnDimCategories = columnDimension.getCategories();
          final List<String> measureCategories = measureDimension.getCategories();
  
          for (final String measureCategory : measureCategories) {
              final Number[] values = new Number[columnDimCategories.size() - 1];
              for (int i = 0; i < columnDimCategories.size() - 1; i++) {
                  nav.where(columnDimension, columnDimCategories.get(i));
                  final CrosstabNavigator<Number> where = nav.where(measureDimension, measureCategory);
                  final Number value = where.safeGet(null);
                  if (!columnDimCategories.get(0).equals(BooleanAnalyzer.VALUE_COMBINATION_COLUMN_FREQUENCY)) {
                      values[i] = value;
                  }
              }
  
              final CrosstabNavigator<Number> where =
                      nav.where(columnDimension, BooleanAnalyzer.VALUE_COMBINATION_COLUMN_FREQUENCY);
              final Number frequency = where.safeGet(null);
              final Number frequencyVal = frequency != null ? frequency : 0;
              final ValueCombination<Number> valComb = new ValueCombination<>(values);
              final Number combination = valueCombMapList.get(valComb);
              if (combination == null) {
                  valueCombMapList.put(valComb, frequencyVal);
              } else {
                  final Number newValue = CrosstabReducerHelper.sum(combination, frequencyVal);
                  valueCombMapList.put(valComb, newValue);
              }
          }
      }
docs: Gather the sum of all possible value combinations of the partial
  crosstabs
  
  @param valueCombMapList
  @param partialCrosstab