er. These
        representations are semantically equivalent.  If a property has an
        array value, the absence of any items in that array MUST be
        represented by omitting the property entirely or by setting the value
        to null. The appropriate interpretation of an omitted or explicitly
        null value is that no value has been assigned as opposed to the view
        that the given value is empty or nil.
      </p>

      <p>
        An <dfn>Activity Streams
        Document</dfn> is a