anchor.diff:  --------------------------------------------------
   {
     "set": {
  -    "field": "{{service}}"
  +    "field": "{{service}}",
       "value": "{{code}}"
     }
   }
  
positive.code: --------------------------------------------------
  {
    "set": {
      "field": "received"
      "value": "{{_ingest.timestamp}}"
    }
  }
  
negative.code: --------------------------------------------------
  {
    "set": {
      "field": "field_c",
      "value": "{{field_a}} {{field_b}}"
    }
  }
  