@abc.abstractmethod<EOL><INDENT>def available(self):<DEDENT>
return<EOL>
Gets the number of ``bytes`` available for retrieval.

        The number returned by this method may be less than or equal to
        the total number of ``bytes`` in this stream.

        :return: the number of ``bytes`` available for retrieval
        :rtype: ``cardinal``
        :raise: ``IllegalState`` -- this stream has been closed

        *compliance: mandatory -- This method must be implemented.*