SiSU 4.2.0

* syntax additions (introduced for 5.0.12) now relevant also for v4
  * ocn (object numbering) on, off, text block using "--~#" "---#" "--+#"
  * grouped text, open, close of blocks using tics "```"

* syntax add
  * switch ocn off and on for a block of content, line containing only
    off: "--~#" (omit headings where possible) "---#" on: "--+#"
  * alternative open and close for blocks using line starting with "```", 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
  [ note further refinements will be required;
    use of tildes "~~" considered but more problematic, tics not used elsewhere]

* increase whitespace (between paragraphs)
* [review spacing (generally) further]

SiSU 4.1.12