anchor.diff:        "ratings": [
           {
             "rating": 1.5,
  -          "_type": "page",
  +          "_type": "tweet",
             "_id": "13736278",
  -          "_index": "enwiki_rank"
  +          "_index": "twitter"
           },
           {
             "rating": 1,
  -          "_type": "page",
  +          "_type": "tweet",
             "_id": "30900421",
  -          "_index": "enwiki_rank"
  +          "_index": "twitter"
           }],
         "params": {
           "query_string": "JFK",                          <2>
  -        "wiki_field": "opening_text"                    <2>
  +        "field": "opening_text"                         <2>
         }
       }],
       "metric": {
  
positive.code: 
  [source,js]
  -----------------------------
  GET /index/type/_rank_eval
  {
      "requests": [
      {
  
negative.code:         "request": {
              "query": {
                  "match": {
                      "title": {
                          "query": "JFK"}}}},             <2>
          "ratings": [                                    <3>
          {
            "rating": 1.5,                                <4>
            "_type": "tweet",                             <5>
            "_id": "13736278",                            <6>
            "_index": "twitter"                           <7>
          },
          {
            "rating": 1,
            "_type": "tweet",
            "_id": "30900421",
            "_index": "twitter"
          }],  
        "summary_fields": ["title"]                       <8>
      }],
  