json_schema: {
      "$schema": "http://json-schema.org/draft-04/schema#",
      "additionalProperties": false,
      "type": "object",
      "properties": {
          "expression": {
              "title": "Expression",
              "description": "What to evaluate; JSON paths are allowed, wrapped in {{this.is.a.path}}",
              "type": "string",
              "minLength": 1
          }
      },
      "required": [
          "expression"
      ]
  }