anchor.diff:      columns: <50% =0% <50%
   
   |<side>_<position>_content^[3,4]^
  -|Quoted string
  +|<<quoted-string,Quoted string>>
   |footer:
     verso:
       center:
  
positive.code:   - Color as hex string (e.g., #ffffff)
    - Image path
    - Enumerated type (where specified)
  * Null (clears any previously assigned value)
    - _empty_ (i.e., no value specified)
    - null
  
negative.code: 3+|[#key-prefix-menu]*Key Prefix:* <<key-prefix-menu,menu>>
  
  |caret_content
  |<<quoted-string,Quoted string>> +
  (default: ' \u203a ')
  |menu:
    caret_content: ' > '
  