* ocn (object numbering) on, off, text block using "--~#" "---#" "--+#"
* grouped text, open, close of blocks using tics "```"

* e.g.
  open code block:  "``` code"
  close code block: "```"
  start poem:       "``` poem"
  end poem:         "```"
* remove need for an empty line between opening & closing of a text block

* switch ocn off & on for a block of content, line containing only
  off: "--~#" (omit headings where possible) "---#" on: "--+#"

* output structure by, add monolingual alt for :filetype & :filename, step 1
* output structure by, monolingual alternative for :filetype or :filename
   without language code, if document is in default language
  * set a default language, 'en' or as specified
  * set in rc file or from command line instruction
    * command line e.g.
      sisu -hv --monolingual --by-filename sisu_markup.sst
      sisu -3v --monolingual --by-filetype sisu_manual.ssm
    * sisurc.yml lingual: 'mono' | 'multi' lingual: 'mono'
  * note: output by :filetype is roughly equivalent to what monolingual output
    by :language would be
  * allow command line setting of default language using language code
    --default-language='en' --default-lang-en if used together with
    --monolingual and --by-filetype or --by-filename the selected default
    language document will not have a language code in the output filename
* output structure by, monolingual alternative, documentation
* cgi helper script, sample search form, single form, monolingual option
  * single cgi form, (with different internal variables set for output types)