Activity Streams 2.0 document is either an
      <code><a>Object</a></code> or a <code><a>Link</a></code>. All other types
      defined in the Activity Vocabulary, as well as all extension types, are
      derived from these two base types.
    </p>

    <p>
      A JSON object in the Activity Streams 2.0 document is a
      <code><a>Link</a></code> if either: (a) the object contains a
      <code>type</code> property whose value includes "<code>Link</code>" or
      (b) any of the types included in the va