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// ways of matching an originator; by user, group, or service account.
// +union
message Subject {
  // `kind` indicates which one of the other fields is non-empty.
  // Required
  // +unionDiscriminator
  optional string kind = 1;

  // `user` matches based on username.
  // +optional
  optional UserSubject user = 2;

  // `group` matches based on user group name.