      <p>
        In Activity Streams 2.0, an "extension" is any property, activity,
        actor or object type not defined by the
        <a href="https://www.w3.org/TR/activitystreams-vocabulary/">Activity Vocabulary</a>.
        Consuming implementations that encounter unfamiliar
        extensions MUST NOT stop processing or signal an error and MUST
        continue processing the items as if those properties were not
        present. Note that support for extensions can vary across
        implementa