anchor.diff:  }'
   --------------------------------------------------
   
  +In addition to `_source`, the following variables are available through
  +the `ctx` map: `_index`, `_type`, `_id`, `_version`, `_routing`,
  +`_parent`, `_timestamp`, `_ttl`.
  +
   We can also add a new field to the document:
   
   [source,js]
  
positive.code: }'
  --------------------------------------------------
  
  We can also add a tag to the list of tags (note, if the tag exists, it
  will still add it, since its a list):
  
  [source,js]
  
negative.code: }'
  --------------------------------------------------
  
  [float]
  === Updates with a partial document
  
  The update API also support passing a partial document,
  which will be merged into the existing document (simple recursive merge,
  