def to_fdf
      fdf = header
      
      @data.each do |key, value|
        if Hash === value
          value.each do |sub_key, sub_value|
            fdf << field("#{key}_#{sub_key}", sub_value)
          end
        else
          fdf << field(key, value)
        end
      end

      fdf << footer
      return fdf
    end
def to_fdf
      fdf = header
      
      @data.each do |key, value|
        if Hash === value
          value.each do |sub_key, sub_value|
            fdf << field("#{key}_#{sub_key}", sub_value)
          end
        else
          fdf << field(key, value)
        end
      end

      fdf << footer
      return fdf
    end
generate FDF content