anchor.diff:  `_id`::
   (string) Document `_id` for the feature's document.
   
  +`_index`::
  +(string) Name of the index for the feature's document.
  +
   `<field>`::
   Field value. Only returned for fields in the `fields` parameter.
   ======
  
positive.code:         },
          "properties": {
            "_id": "4",
            "name": "Amsterdam Centre for Architecture",
            "price": 0
          },
  
negative.code:         },
          "properties": {
            "_id": "1",
            "_index": "museums",
            "name": "NEMO Science Museum",
            "price": 1750
          },
  