format_internal

# File lib/net/imap.rb, line 1433
      def format_internal(data)
        case data
        when "*"
          return data
        when Integer
          ensure_nz_number(data)
          if data == -1
            return "*"
          else
            return data.to_s
          end
        when Range
          return format_internal(data.first) +
            ":" + format_internal(data.last)
        when Array
          return data.collect {|i| format_internal(i)}.join(",")
        when ThreadMember
          return data.seqno.to_s +
            ":" + data.children.collect {|i| format_internal(i).join(",")}
        else
          raise DataFormatError, data.inspect
        end
      end