anchor.diff:  |page:
     size: Letter
   
  -|numbering-start-at
  +|numbering-start-at ^[5]^
   |title {vbar} toc {vbar} body +
   (default: body)
   |page:
  
positive.code: The `columns` key can also be defined one level up (on `header` or `footer`), in which case the setting will be inherited.
  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`.
  
  IMPORTANT: If you don't specify a height for either the header or footer key, it effectively disables the content at that periphery.
  
  
negative.code: . Target may be an absolute path or a path relative to the value of the `pdf-themesdir` attribute.
  . The margins for `recto` (right-hand, odd-numbered) and `verso` (left-hand, even-numbered) pages are calculated automatically from the margin-inner and margin-outer values.
  These margins and used when the value `prepress` is assigned to the `media` document attribute.
  . The `toc` value only applies if the toc is in the default location (before the first page of the body).
  If the toc macro is used to position the toc, the start-at behavior is the same as if the toc is not enabled.
  
  [#keys-base]
  === Base
  