"indentation_mark": {
  "type": "object",
  "additionalProperties": false,
  "required": [
    "type",
    "attrs"
  ],
  "properties": {
    "type": {
      "enum": ["indentation"]
    },
    "attrs": {
      "type": "object",
      "additionalProperties": false,
      "required": ["level"],
      "properties": {}
    }
  }
}

 "type": {
  "enum": ["indentation"]
}

 "attrs": {
  "type": "object",
  "additionalProperties": false,
  "required": ["level"],
  "properties": {
    "level": {
      "type": "number",
      "minimum": 1,
      "maximum": 6
    }
  }
}

 "level": {
  "type": "number",
  "minimum": 1,
  "maximum": 6
}