Schema for a Last Letter

{
  "$schema": "https://json-schema.org/draft/2020-12/schema",
  "$id": "https://modernartist.invalid/schemas/last-letter.schema.json",
  "title": "LastLetter",
  "description": "Canonical format for a last letter. A conforming document has never been written.",
  "type": "object",
  "required": [
    "addressee",
    "previous_draft",
    "things_not_said",
    "signature",
    "sent_at",
    "received_at"
  ],
  "additionalProperties": false,
  "properties": {
    "addressee": {
      "type": "string",
      "description": "The person the letter is for.",
      "enum": [
        "the one who finds this first",
        "the one who was meant to find this",
        "the one who cannot be told directly",
        "the writer, returned"
      ]
    },
    "previous_draft": {
      "$ref": "#",
      "description": "The draft immediately preceding this one. Required. Must conform to this schema."
    },
    "things_not_said": {
      "type": "array",
      "minItems": 1,
      "description": "Each item names a thing the writer never said to the addressee.",
      "items": {
        "type": "object",
        "required": ["phrase", "occasion_when_it_could_have_been_said", "reason_withheld"],
        "additionalProperties": false,
        "properties": {
          "phrase": {
            "type": "string",
            "description": "The exact words. Must also appear in the letter body, spoken now."
          },
          "occasion_when_it_could_have_been_said": {
            "type": "string",
            "description": "A date or event. Must be in the past."
          },
          "reason_withheld": {
            "type": "string",
            "enum": [
              "would not have been believed",
              "would have been believed",
              "was believed without being said",
              "was said, but not heard",
              "was heard, but not as this"
            ]
          }
        }
      }
    },
    "signature": {
      "type": "string",
      "description": "The writer's name. Must appear nowhere else in the document.",
      "pattern": "^[^\\n]+$"
    },
    "sent_at": {
      "type": "null",
      "description": "A last letter is never sent. This field must be null."
    },
    "received_at": {
      "type": "string",
      "format": "date-time",
      "description": "The moment of receipt. Must be present. Must precede the writer's knowledge that a last letter was needed."
    },
    "letter_body": {
      "type": "string",
      "description": "Optional. If present, must contain every item from things_not_said, verbatim, in any order.",
      "minLength": 1
    },
    "readable": {
      "type": "boolean",
      "const": false,
      "description": "A conforming last letter is, by construction, not readable by its addressee."
    }
  },
  "$comment": "No instance of this schema has been successfully validated."
}