@type t() ::
  integer()
  | :EXTERNAL_REFERENCE_TYPE_OTHER
  | :EXTERNAL_REFERENCE_TYPE_VCS
  | :EXTERNAL_REFERENCE_TYPE_ISSUE_TRACKER
  | :EXTERNAL_REFERENCE_TYPE_WEBSITE
  | :EXTERNAL_REFERENCE_TYPE_ADVISORIES
  | :EXTERNAL_REFERENCE_TYPE_BOM
  | :EXTERNAL_REFERENCE_TYPE_MAILING_LIST
  | :EXTERNAL_REFERENCE_TYPE_SOCIAL
  | :EXTERNAL_REFERENCE_TYPE_CHAT
  | :EXTERNAL_REFERENCE_TYPE_DOCUMENTATION
  | :EXTERNAL_REFERENCE_TYPE_SUPPORT
  | :EXTERNAL_REFERENCE_TYPE_DISTRIBUTION
  | :EXTERNAL_REFERENCE_TYPE_LICENSE
  | :EXTERNAL_REFERENCE_TYPE_BUILD_META
  | :EXTERNAL_REFERENCE_TYPE_BUILD_SYSTEM