number of <code>Activity</code> types that are common
        to many social Web applications. This specification stops short of
        defining semantically specific properties for most of these activities.
        External vocabularies can be used to express additional detail not
        covered by the Activity Vocabulary.
      </p>

      <p>
        While implementations are free to introduce new types of Activites
        beyond those defined by the Activity Vocabulary, interoperability
        issue