def available(self):
return self._payload_list.available()<EOL>
Gets the number of elements available for retrieval.

        The number returned by this method may be less than or equal to
        the total number of elements in this list. To determine if the
        end of the list has been reached, the method ``has_next()``
        should be used. This method conveys what is known about the
        number of remaining elements at a point in time and can be used
        to determine a minimum size of the remaining elements, if known.
        A valid return is zero even if ``has_next()`` is true.

        This method does not imply asynchronous usage. All OSID methods
        may block.

        return: (cardinal) - the number of elements available for
                retrieval
        *compliance: mandatory -- This method must be implemented.*
        *implementation notes*: Any errors that may result from accesing
        the underlying set of elements are to be deferred until the
        consumer attempts retrieval in which case the provider must
        return a positive integer for this method so the consumer can
        continue execution to receive the error. In all other
        circumstances, the provider must not return a number greater
        than the number of elements known since this number will be fed
        as a parameter to the bulk retrieval method.