letter_references = {
    "$schema": "http://json-schema.org/draft-07/schema#",
    "description": "list of letter notification references",
    "type": "object",
    "title": "references",
    "properties": {
        "references": {"type": "array", "items": {"type": "string", "pattern": "^[0-9A-Z]{16}$"}, "minItems": 1},
    },
    "required": ["references"],
}
