anchor.diff:  --------------------------------------------------
   {
     "set": {
  -    "field": "field_c"
  +    "field": "field_c",
       "value": "{{field_a}} {{field_b}}"
     }
   }
  
positive.code: --------------------------------------------------
  {
    "set": {
      "field": "_id"
      "value": "1"
    }
  }
  
negative.code: --------------------------------------------------
  {
    "set": {
      "field": "my_field",
      "value": 582.1
    }
  }
  