anchor.diff:  === Detecting noop updates
   
   If `doc` is specified its value is merged with the existing `_source`.
  -By default updates that don't change anything detect that they don't change anything and return "result": "noop" like this:
  +By default updates that don't change anything detect that they don't change anything and return `"result": "noop"` like this:
   
   [source,js]
   --------------------------------------------------
  
positive.code: // TEST[continued]
  
  And, we can even change the operation that is executed.  This example deletes
  the doc if the `tags` field contain `green`, otherwise it does nothing
  (`noop`):
  
  [source,js]
  
negative.code: // TEST[continued]
  
  In addition to `_source`, the following variables are available through
  the `ctx` map: `_index`, `_type`, `_id`, `_version`, `_routing`,
  and `_now` (the current timestamp).
  
  We can also add a new field to the document:
  