json_schema: {
      "$schema": "http://json-schema.org/draft-04/schema#",
      "definitions": {
          "simple-extension": {
              "type": "object",
              "properties": {
                  "url": {
                      "type": "string"
                  },
                  "value": {
                      "type": "string"
                  }
              }
          },
          "complex-extension": {
              "items": {
                  "type": "object",
                  "properties": {
                      "extension": {
                          "$ref": "#/definitions/simple-extension"
                      }
                  }
              }
          }
      },
      "properties": {
          "simple-extension": {
              "$ref": "#/definitions/simple-extension"
          },
          "complex-extension.items": {
              "$ref": "#/definitions/complex-extension/items"
          }
      },
      "required": [
          "simple-extension",
          "complex-extension.items"
      ]
  }