anchor.diff:  request,  after aggregations have already been calculated. Its purpose is
   best explained by example:
   
  -Imagine that you are selling shirts, and the user has specified two filters:
  +Imagine that you are selling shirts that have the following properties:
  +
  +[source,js]
  +-------------------------------------------------
  +PUT /shirts
  +{
  +    "mappings": {
  +        "item": {
  +            "properties": {
  +                "brand": { "type": "keyword"},
  +                "color": { "type": "keyword"},
  +                "model": { "type": "keyword"}
  +            }
  +        }
  +    }
  +}
  +
  +PUT /shirts/item/1?refresh
  +{
  +    "brand": "gucci",
  +    "color": "red",
  +    "model": "slim"
  +}
  +------------------------------------------------
  +// CONSOLE
  +// TESTSETUP
  +
  +
  +Imagine a user has specified two filters:
  +
   `color:red` and `brand:gucci`.  You only want to show them red shirts made by
   Gucci in the search results.  Normally you would do this with a 
   <<query-dsl-bool-query,`bool` query>>:
   
   [source,js]
   --------------------------------------------------
  -curl -XGET localhost:9200/shirts/_search -d '
  +GET /shirts/_search 
   {
     "query": {
       "bool": {
  
positive.code:     }
    }
  }
  '
  --------------------------------------------------
  <1> Returns the most popular models of red shirts by Gucci.
  
  But perhaps you would also like to tell the user how many Gucci shirts are
  
negative.code:     }
    }
  }
  --------------------------------------------------
  // CONSOLE
  
  However, you would also like to use _faceted navigation_ to display a list of
  other options that the user could click on.  Perhaps you have a `model` field
  