anchor.diff:  
   [source,yaml]
   ----
  +extends: default
   footer:
     columns: =100%
     recto:
  
positive.code: 
  [source,yaml]
  ----
  footer:
    recto:
      right:
  
negative.code:       content: '{page-number}'
  ----
  
  In the last two examples, the recto and verso both have the same content.
  In this case, you can reduce the amount of configuring using a YAML reference.
  For example:
  
  [source,yaml]
  ----
  extends: default
  footer:
    columns: =100%
    recto: &shared_footer
      center:
        content: '{page-number}'
    verso: *shared_footer
  ----
  
  The `&shared_footer` assigns an ID to the YAML subtree under the `recto` key and the `*shared_footer` outputs a copy of it under the `verso` key.
  This technique can be used thoughout the theme file as it's a core feature of YAML.
  
  ==== Attribute References
  
  