code: def process_aggregation
        File.new(infile).each_with_index do |line, index|
          result = col_filter.process(row_filter.process(line.chomp, row: index))
          unless result.nil? or result.empty?
            if heading.empty? and not headerless
              heading << result.split(';')
              next
            else
              @sum_col = [result.split(';').size, sum_col].max 
            end
            key_values[result]  += 1
            sums[sum_col_title] += 1
          end
        end
        heading.flatten!
        heading[sum_col] = sum_col_title
      end
docs: Process the aggregation of the key values. The result will be written to
   _outfile_