Returns a set of the available versions.

        :returns: A set of integers giving the available versions.

def available(self):
        """
        Returns a set of the available versions.

        :returns: A set of integers giving the available versions.
        """

        # Short-circuit
        if not self._schema:
            return set()

        # Build up the set of available versions
        avail = set(self._schema.__vers_downgraders__.keys())
        avail.add(self._schema.__version__)

        return avail