{
  "$schema": "http://json-schema.org/draft-06/schema#",
  "type": "object",
  "properties": {
    "subject": {"type": "string", "format": "uri"},
    "vote": {"type": "number", "enum": [-1, 0, 1]},
    "createdAt": {"type": ["number", "null"]}
  },
  "required": ["subject", "vote"]
}