anchor.diff:  |footer:
     vertical_align: top
   
  -|<side>_columns^[3]^
  +|<side>_columns^[4]^
   |Column specs triple +
   (default: _not set_)
   |footer:
     verso:
       columns: <50% =0% <50%
   
  -|<side>_<position>_content^[3,4]^
  +|<side>_<position>_content^[4,5]^
   |<<quoted-string,Quoted string>> +
   (default: '\{page-number}')
   |footer:
  
positive.code: |===
  . The background color spans the width of the page, as does the border when a background color is specified.
  . If the height is not set, the running content at this periphery is disabled.
  . `<side>` can be `recto` (right-hand, odd-numbered pages) or `verso` (left-hand, even-numbered pages).
  Where the page sides fall in relation to the physical or printed page number is controlled using the `pdf-folio-placement` attribute (except when `media=prepress`, which implies `physical`).
  . `<position>` can be `left`, `center` or `right`.
  
negative.code: |footer:
    line_height: 1.2
  
  |padding^[3]^
  |<<measurement-units,Measurement>> {vbar} <<measurement-units,Measurement[top,right,bottom,left]>> +
  (default: 0)
  |footer:
  