{
  "$schema": "http://json-schema.org/draft-03/schema#",
  "id": "http://www.popoloproject.com/schemas/identifier.json#",
  "title": "Identifier",
  "description": "An issued identifier",
  "type": "object",
  "properties": {
    "identifier": {
      "description": "An issued identifier, e.g. a DUNS number",
      "type": "string",
      "required": true
    },
    "scheme": {
      "description": "An identifier scheme, e.g. DUNS",
      "type": ["string", "null"]
    }
  }
}