anchor.diff:  
   [source,js]
   --------------------------------
  -{
  +GET /index/type/_rank_eval
  +{
  +    "requests": [
  +    {
  +        "id": "JFK query", 
  +        "request": { "query": { "match_all": {}}},
  +        "ratings": []  
  +    }],
       "metric": {
           "reciprocal_rank": {}
       }
   }
   --------------------------------
  -
  +// CONSOLE
   
   === Normalized discounted cumulative gain
   
  
positive.code:    }
  }
  --------------------------------
  
  <1> For graded relevance ratings only ratings above this threshold are
  considered as relevant results for the given query. By default this is set to 1.
  
negative.code:           "_id": "30900421",
            "_index": "enwiki_rank"
          }],  
        "summary_fields": ["title"]                       <8>
      }],
      "metric": {                                         <9>
        "reciprocal_rank": {}
     }
  